Why isn't taxability of benefits indexed?

@Texconsin To understand why SS benefits are taxable at 50% and 85%, you need to review the history at ssa.gov. It is lengthy and may provide the reason(s) why the thresholds have not been indexed. As I recall, SS amendments in 1983 and 1993 put in place the 50% and 85% thresholds, respectively, for the higher income folks. Initially, only a small percentage reported SS benefits as taxable income.Today, about 56% of the folks pay some amount of tax on their SS benefits. I am providing a link to the 2020 Census https://www.census.gov/content/dam/Census/library/visualizations/2021/demo/p60-273/figure1.pdf wherein income is "sliced and diced" for selected characteristics. For the 65 years and older characteristic, the median income is $46,360. As I understand "median", this is the middle. Half are above and half are below. Based on that statistic, I suspect it is not a "hot button" issue for Congress. In another posting, someone commented that as long as the SS Trust Fund is projected to deplete around 2034-2035, SS benefits will remain taxable. I agree.  

@Texconsin 

Since AARP advocates supporting proposals to strengthen Social Security and a lot of the income to the Trust Funds are currently coming from these beneficiary taxes on their SS benefits - I doubt this is the proper time to be talking about reducing revenues into the Trust Funds by lifting or indexing the income limits.

Social Security Trust Fund Data 1957 to present

However, at the state level, I believe they have been successful over the years in convincing many states to stop taxing these benefits. 

There are some House proposals to "fix" Social Security that contain provisions to raise the income limits to which these benefit taxes would apply.  But don't hold your breath cause again this would reduce revenues to the program and would have to be made up somewhere else.

Under current law, single tax filers with combined “income” (approximately equal to adjusted
gross income plus non-taxable interest income and one-half of their Social Security benefit)
greater than $25,000 may have to pay income tax on up to 50 percent of their Social Security
benefits. If combined “income” exceeds $34,000, up to 85 percent of benefits may be taxable.
Currently this income tax revenue on SS benefits is disbursed as follows:

• For taxing up to 50 percent of Social Security benefits is credited to the
  OASI and DI Trust Funds.
• The additional income tax revenue derived from taxing benefits in
  excess of 50 percent, up to 85 percent, is credited to the HI Trust Fund.

The process is similar for joint tax filers, with $32,000 and $44,000 thresholds applying for possible taxation of up to 50 percent or 85 percent of the Social Security benefits, respectively. 

The following changes have been suggested in the following bill (pdf version of the SS Actuary report on the bill)

Social Security 2100: A Sacred Trust” Act, introduced on October 26, 2021 by Chairman John Larson 

Section 104. Replace the current-law thresholds for federal income taxation of OASDI
benefits with a single set of thresholds at $35,000 for single filers and $50,000 for joint filers
for taxation of up to 85 percent of OASDI benefits, effective for tax years 2022 through 2026.
Revert to current-law specifications for tax years 2027 and later.

Under the proposal, both sets of the current-law thresholds would be replaced with a single set of
thresholds, $35,000 and $50,000 for single and joint filers, respectively, for taxing up to 85 percent of OASDI benefits, effective for tax years 2022 through 2026. For tax years 2022
through 2026, the amount of revenue from taxation of OASDI benefits that would be allocated to
the HI Trust Fund would be at the same level as if the current-law computation (in the absence of
this provision) were applied. The net amount of revenue from taxing OASDI benefits, after the
allocation to HI, would be allocated to the combined OASI and DI Trust Funds. All
specifications for taxing Social Security benefits would revert to current law for tax years 2027 and later.

Again, I would not hold my breath - these income limits for taxes on benefits have been in place for a VERY LONG TIME.  We have all kinds of ways that we can plan for retirement and thus we can also plan for these taxes and help out the program.

@Texconsin   With regard to indexing the thresholds, I  found what I believe was the first Bill that addressed indexing the SS thresholds. I included the link in my reply to Gail1. That Bill was introduced August 6, 1993 or 28 years ago. I suspect there were additional Bills with variations since then. Further, you can spend hours if not days, if you have the time and patience, reviewing the proposed Bills and info at congress.gov. I found an article that provides some insight into how Bills are initiated and the process which a Bill becomes law. https://newhealthcarebillfacts.com/how-many-bills-are-introduced-in-congress-each-year/ Although the article is titled new healthcare bills, it indicates that the 115th Congress introduced over 10,000 Bills between 2017 and 2019 and only 443 were signed into law. I guess some folks would state our Congressmen and women are very busy. Other folks will state they waste a lot of time and money producing Bills that go nowhere. We can only guess what issues the AARP will support. I suspect the issues will be broad based that affect many folks not just the higher incomes. 

Just for fun, I entered the $34 K and $44 K thresholds into a Future Value calculator and used 2% as the interest factor for 28 years with compounding just once per year and the results are $59 K and $77 K ,respectively for 2021. I rounded the fractional dollars up or down to the next thousand. I am guessing this indexing, if implemented, would have cost the SS Programs Billions over 28 years.