Well, there's a lotta reasons for that. First and foremost is that the budgets for AAA titles nowadays are getting higher and higher. The majority of those are American games.
I'd argue that - in terms of major releases - there are four notable and region-based tones to each culture.
(Disclaimer: Yes, these are gonna be generalisations, and there will be plenty of exceptions, however when viewed as one culture in the macro sense, you will see that they are common traits)
America is the home of the blockbuster. Bigger is better. The land of the wild west. Guns, explosions, action heroes. They also wanna say summat about society on occasion.
Europe tends to focus on more introspective games, and tend to be quite experimental.
Japan is Japan. It's a bit of a mishmash of culture, due in part to the post-WW2 US occupation. Japanese games, to me, are quite playful on the whole, and can be quite bizarre. To us, at least.
Then there's us. The Brits. We're all about humour. Go and look at all of our major releases over the past 30 years, disregard any licensed games like the Arkham series, SEGA racing, Aliens, sports games, etc and you'll see the good ol' British humour run amok.
The thing is, as time has progressed, we've kinda slipped down the pecking order. Well, maybe not down the pecking order exactly, but there are huge, notable differences between the budgets UK-based studios work off of, and those of our American counterparts. They unequivocally run the triple-A market right now.
I'm sure Japanese games may sell more on the whole, I don't know the figures, but across Europe and the US, it's undeniable that American developers/publishers gain the most exposure. They just spend more on marketing. That's their way, and that's what I mean when I say they run the market.
Anyway, even our crown jewel Rockstar are becoming more Americanised as time goes on. It makes sense. Hell, the Housers live over there, and it's practically their company. Well, it was, I think they're both leaving now, but still... That company has British roots, but it's effectively been pre-approved for US citizenship.
So when you say that games are a lot more serious now, they're not. You're just being exposed to more games that have a seemingly "more serious" tone. They're the blockbusters. And sure, blockbusters may have some quips and killer one-liners, but it's not exactly "direct some mice to rocket ships to escape space cats".
There's a much deeper analysis to be made here, but I'm tired and meant to be doin shit atm, so you'll just have to wait on someone to do a long-form video essay on YouTube or summat.