Is it? Even outside of sales tax, it's additional accounting overhead to handle the withholding and filing for states you aren't in. No idea how much, I would imagine it's dependent on the systems in use, but the organizational incentives for allowing and enabling it are probably horribly misaligned for many companies.
It's a hassle but not an huge one, and any company that is in a population center that straddles a state border already deals with this (think New York, Chicago, Portland, Minneapolis/St Paul) - once you can handle two states handling additional isn't that painful.
More importantly the COST associated is fixed per employee roughly - having an employee in another state doesn't usually change the amount of tax paid, etc - just who it's paid to.
This won't be entirely true, but it's close enough - and for companies with outsourced HR (think: ADP) it's even easier.
Any company big enough to have a significant number of employees that this applies to would be big enough to eat the cost (and probably already has offices/etc in multiple states as is).
