Sure, over the last 40+ years wage growth has not grown significantly. However, non-monetary compensation in the form of benefits, like health insurance premiums paid by your employer, has grown. This means that overall compensation has grown at a greater rate than wages. Since SS only taxes wages, a significant amount of the increased compensation is not being taxed and included in SS contributions. This contributes to the shortfall.
The SSA has a write up on this that I thought was very educational:
https://www.ssa.gov/policy/docs/ssb/v73n1/v73n1p83.html
For illustration purposes - when your insurance premiums go up and your employer pays it, it's similar to getting a raise equal to the value of the premium increase. Except that you never see the money because the employer is spending it for you. However because it's being spent on a benefit, you never pay SS taxes on it and thus the SS fund get no SS contribution for your future needs.
To overly simplify, you're basically giving up future SS benefits to pay for annual employee benefits packages in the now. This particularly hurts the middle class because they're not making enough in wages to adequately fund their retirements on their own and they're not funding their SS accounts to the highest possible level because their pay raises are spent on benefits. It's a healthcare argument that actually impacts far more than I'd previously realized.