Almost everyone shown exposed to gamma rays in the Marvel U gets powers, too, but people don’t run out and expose themselves to them, either (and it’s much easier, to do so, too).
Even ignoring that most people CAN’T get access to cosmic rays, gamma bombs, or whatever, there’s the fact that, even those who’ve gotten the powers don’t always get unalloyed powers, some of them get powers paired with horrible mutations.
Ben Grimm - Rocky hide, ugly as sin. Still not to bad.
The U-Foes - it nearly KILLED them - and that was just the actual effects of their powers when they failed to get control of them immediately.
Bruce Banner - great superpowers…creation of a second personality that’s sometimes evil, sometimes semi-mindless, sometimes hates the original you, and comes out under ever changing circumstances (sometimes at night, sometimes when your emotions get overwhelming, sometimes you just go away and the secondary personality is the only one, etc).
Abomination - ugly as sin.
The Leader - big, ugly brain…not a bad tradeoff, all told, still, not something everyone would want. Besides, he seemed a non-evil enough sort of dude beforehand, so perhaps his criminal tendencies are gamma-induced.
This is assuming, of course, you get a strictly comic-book result, rather than the real world result of a slow, painful death.
Any easily replicated, or common, way of gaining superpowers in the Marvel U has a tendency to create bad side effects in some, many, or most individuals.
Super Soldier Serum only worked properly on Captain America…other subjects suffered physical and mental problems.
Mutations can be harmful as often as they are beneficial - and simply being a mutant is problematic to begin with.
Goblin Serum almost always induced insanity.
Even power-suits have backfired spectacularly at times! (Iron Man: Hypervelocity is a good example of this.)
It’s too much of a crap-shoot, even for people with access to the means.