The tax laws of the UK are over 16000 pages long and yet those seem to be handled quite well by both the authorities and people.

Employers may deal with filing tax returns for their workers - to an extent - but this is information the government already has afaik. A long criticism of the US has been this same transparency has been offered to their citizens but they're still expected to file their own tax returns too and make the figures match up to what the irs have.

If the IRS expect it to cost so much to build a system of their own, why don't they just buy a company that can already do the accountancy and go from there (or get banks to do it since they'll have a lot of the information already too).