Well we can. Someone owns the robots (or owns shares in the corporation that owns the robots). Just change the tax code to make Social Security tax apply to all income, including capital gains, with no upper limit.
The reason why capital income is not subject to social isurance taxes is because it is fundamentally an insurance scheme. People who earn mostly wages need social insurance in retirement (when they would not be physically able to work), so they (have to) pay it, while people who earn mostly capital income do not need it (as capital income does not depend on personal health / fitness to work).
Therefore extending social insurance taxes to capital income without extending pension payments to capital earners would be unfair. Local politicians tried something similar - remove caps on social insurance, while keeping pensions capped / strongly sublinear, and it was struck down by constitutional court.
Perhaps reasonable solution would be to split pensions to tax-based UBI, and smaller contribution based 'linear' pensions.
