I think the main issue for the companies is that power adapters have a nearly unlimited lifespan in comparison to lithium batteries, so it would be less profitable for them to sell you a direct attached power adapter than a bunch of batteries and a charger where you have to keep crawling back to them when the batteries inevitably give out in three years.

It would be trivial to design a blank battery attachment with a DC jack, and just have it hooked up to what is essentially a beefed up laptop charger. There are plenty of applications where a corded tool is perfectly adequate and even superior to cordless tools, so the fact that none of the manufacturers have it as an option hints that it was a business decision as opposed to merely an oversight.

Imagine having a power brick for your battery drill. Truly the worst of both worlds.