Well, it’s a multi prong issue; there isn’t a single cause, and not everyone shares the same combination of causes.

First is the obvious. Age related eye and coordination issues. As we age, some things don’t keep it up like other traits can. And, worse, age related experience only counters it if the person applies said experience.

Second, you run into issues with the plumbing, as someone said in other comments. The prostate is a hell of an organ. But when it gets swollen, the stream gets sputtery. You also run into user error where you have to hold the thing different than when you’re younger because the elasticity and thickness of skin changes. It’s easier to accidentally apply pressure to the urethra. The typical two or three finger grip can apply more pressure than is good when the skin isn’t as resilient.

Third, the ol’ give-shitter runs down. Spoons, baby. You run out of them. The effort of paying attention just isn’t as bearable, depending on what else is melting your spoons down. Which applies to cleaning up after yourself as well. Which also means that remembering to behave better when you aren’t at home where you’re the person inconvenienced can be a problem.

Now, the answer to all of those is to sit the fuck down, but the folks now into their seventies and eighties had a much less free concept of masculinity. In other words, they were fed the unhealthy concept that doing things the way women do them is inherently unmasculine. That isn’t true for everyone, but actual boomers, and the generation before them have more resistance to sitting down just because they’re men. Boomers less so than their parents, as the actual baby boomers includes the very same people that fought to change such concepts.

You also have plain assholes, but that isn’t a generation thing; there’s assholes that will spray piss everywhere on purpose of all ages, so it isn’t really a factor for this question.

Legit, I had patients back in the day that would resist sitting down to put on pants because they felt it made them lesser. Not less masculine, just less independent. They’d resist canes and walkers for the same reason, accelerating their reliance on mobility aids. That aspect of seat pissing is also a factor, but like just being an asshole, it isn’t purely a generational thing. Plenty of young guys fight against anything other than standing to pee because being able to do so becomes a symbol of physical capability. If you have to sit to pee, you’re fucked is the idea, no matter how false.

I had to lay down the law with my own dad a couple of years ago. Got tired of needing to use the toilet and sitting in piss, or walking in it, or seeing it drip. In his case, he didn’t even realize he was missing, but he still didn’t like the idea. He also got cataract surgery after that and it improved his aim back to an acceptable level when he does stand while peeing.

For real though my fellow men; sit to fucking pee. Splashing toilet water isn’t a good thing, and that’s going to happen no matter what else you do.