en.wikipedia.org/wiki/Coolidge_effect
So it's not the actual sex, more the ego boost that they got a woman into bed?
No, it's certainly the actual sex too. Or rather, it's the actual sexual arousal. People aren't motivated to shag someone by a dispassionate appraisal of how good the last time was (and when considering shagging a stranger, there's no "last time" to go on anyway). They're motivated by how horny they feel. And men seem to feel a increased boost of horniness when aroused by someone new.
Why are most women not like this? Genuine question?
As caramel said and the link refers to, it is known to be true of women as well, just not as strongly as for men. Noone really knows why. Evolutionary psychologists have various suggestions mostly based on the differing consequences for men and women of each complete act of sex, pregnancy and child rearing, but that depends how much one believes in evolutionary psychology.
I wouldn't start trying to convince yourself that it's all about stuff external to the sex. It seems to be the case that men just feel a huge amount of desire to shag lots of different people (varying by individual, of course). Even men in long term monogamous relationships maintain those relationships despite that conflicting desire; it's not like it just disappears. Hence all the threads on here by women unhappy about their partners' porn habits.