In the US there are many agencies you have to deal with and a lot of paperwork that must be filed. Unlike Canada where you tax based on the work-location and pay everything to the federal government, the US has all sorts of fun rules that mean you could be remitting taxes to multiple state/county/local agencies. And each agency has a different level of technology savvy so you could be faxing/mailing forms to get a particular tax paid.

So calculating gross->net scales nicely, but paying the fed/state/county/local agencies doesn't. And aside from the actual costs of providing the service, it's a tremendous value to the employer. Even at low hourly wages that $25 pays for itself.