Because collective benefits don't work without the collective. Might as well say "If you want roads why don't you just build or pay for them yourself?" "If you want electricity, just build and operate your own power plant"

You can do a whole hell of a lot more with 100 people working together than with 100 individuals acting of their own isolated accords.

But you just said “the overall benefit I receive from increased contribution greatly outweighs having that money in my bank account”. The marginal utility to the government of your cheque only decreases if more people do the same.