Because there is an entire industry of which Intuit is part of that profits off the complexity of current tax system. Simplify the system and profits go down.

HR Block was caught lobbying against simplification of the tax system for this very reason.

Because Congress never votes for a reform of tax codes because the lobbying from Intuit and HR Block et al is too strong, while there's no lobby for the populace at large to say "This sucks."