Most studios without a lot of visible treatment have plenty of material where you can't see it... And the design of the room itself of course - The 'right' room doesn't necessarily need as much trapping as (for example) a rectangle.
To some extent, it's preference -- I don't like "dead" spaces for much of anything - although I'm sure that if I were building a studio today, there would be a small booth somewhere (maybe in the 100 sq ft range) that would be as neutral as possible (exposed studs filled with rockwool, buildups in the corners, etc.).
For "mission critical" areas (mixing / mastering / "real" booths where you're shooting for raw but natural accuracy of the source), "highly controlled but naturally live" is a favorite. I absolutely hated the LEDE craze from back when (although I was guilty of designing an audio complex with a semi-LEDE control room myself because I didn't realize how much I hated it yet).
This room (for example) has the equivalent of roughly 36 2'x4'x4" 703/705-type traps. It's very well controlled but at the same time it doesn't sound alien to the ears (anyone who's ever made the mistake of covering walls with foam knows what I mean by "alien to the ears" which usually translates to unnatural sounding recordings as well).
Sticking to the basics -- You will never, ever regret getting the low end under control before you concentrate on anything else. You will never control the low end perfectly -- But getting it in decent shape can very easily yield a room that needs no additional treatment anyway.