In real life? Sure, they wouldn't be.
But in the vast majority of D&D groups and groups in a lot of other games, realistically they are basically doing as a form of demented benevolence.
Not sure I'd equate donating to churches in D&D with benevolence given how awful most gods are, but I've absolutely seen PCs drop huge amounts of money on stuff like building/supporting orphanages, cast healing spells on villagers with no expectation of pay and so on. I think that's routine in my experience. Ironically it's often PCs who talk a lot of smack about how they're "just mercenaries" who actually do it. This is something that comes up in fiction from time to time too - just look at the recent "Sentenced to be a Hero" anime, where the hilariously-named Xylo Forbartz tries to pass himself off as a hard-bitten selfish badass in the Snake Plissken mode, but as pointed out by other characters, he's anything but that in terms of what he does (as opposed to the persona he adopts), and virtually everything he does is about helping innocent or helpless or weak people or minimizing casualties or the the like. Even his "horrific crime" which lead to being made into a Hero (basically someone who can be resurrected and made to fight demons over and over) was (minor spoilers, resolved in like E2 or E3), an act of pure mercy/compassion which cost him everything.
(EDIT: As an aside, Sentenced to be a Hero is interesting in that it's a rare modern example of dark fantasy that definitely isn't grimdark, even though there are tons of signifiers that make you think it's going to be.)
