It comes down to cost, both real (in the case of the publisher) and opportunity (in the case of the artist).
Since you brought up Todd Lockwood, I'll use him as an example.
Paizo's current pay rate for covers starts at about $1200 and goes up based on the "asking price" of a given artist. Because Todd Lockwood is so experienced, such a great artist, and has more art opportunities than he has time to paint, he is not available at Paizo's regular (or even high) cover rate. We can't afford him. Nor can we afford Brom or Tony DiTerlizzi, both of whom have achieved a fair amount of fame and success and are no longer doing much in our industry.
Add in the fact that most RPG cover art is contracted on an all-rights "work for hire" basis (which means the publisher owns the painting and severely limits the artist's ability to reprint or make any additional money off the work short of selling the original), and you're adding a stipulation that many artists just don't want to deal with when they have the option of more lucrative art that they own outright.
So, let's say we could somehow convince Todd Lockwood to work under his pay rate for the fraction of the rights he is used to retaining. And it's worth noting that for most artists, they're willing to do anything if the price is right, so something like this isn't out of the question if I'm willing to bust the bank and sacrifice profit margin for the right cover artist. So Todd's on the (very expensive) cover. Huzzah!
Think he wants to do any interiors? Paizo pays about $600 per full page of color art, with a half-pager getting $300 and so on down. This rate is better than most companies in the industry, but Wizards of the Coast pays a bit more. Many artists will tell you that a full-page illustration takes the same amount of time as a cover. So you do the math. Is it better for the artist to take another $1500+ cover assignment (for which he will probably retain all rights) or do my interior piece for less than half the amount of money?
After a certain point, artists start viewing themselves as "cover" artists, so they may turn down interior work on principle. Sure, you can get them to do interiors, but it's going to cost a LOT more than it will to get another artist to do it, and we're back to sacrificing profit margin. And again, if they're really good, they have other publishers knocking on their door with more lucrative work. Hell, if they're someone like Wayne Reynolds, they're probably moving on to another Paizo cover immediately after they finish the one just before it, so putting him on interiors robs me of the ability to use him on the cover of the next big release.
So, in summary, as I said before: They expensif.
--Erik