I believe age and accessibility has something to do with it.
A lot of teens (And some adult unfortunately) are rather selfish and kind of arrogant. They have yet to understand how important courtesy is. Especially with those special flakes who think they are so special and unique.
Combined this with how easy it is to access internet today, and voila, you have waves of insolent brats online.
Bad seeds always exist, but the rise of internet has made their presence that much more noticeable nowadays.
I agree with what have been said so far, and I would like to add the distancing that happens with online vs "Face-to-face". That are some nice studies about it nowadays.
In a way, people would not say bad things face-to-face when talking with someone else because they feel accountable for what they are saying, which does not happens online.
Of course, that's not the only answer for those acts, but I find it very interesting to point out.
Being a courteous person shows that you undertstand a person from their perspective. I am very respectful when someone tells me something. I appreciate when people help me. It is very courteous of them to help me.
I can totally relate to what you're saying. And I completely agree. A lot of people out there make it difficult for online play. But there are few gems out there that make it so I want to keep going.
If we're talking about courtesy in the context of online communities, I think that it can really depend on the nature of the community itself and that the overall "Vibe" can affect players' attitudes towards one another. Some positive examples include collaborative efforts on r/place (The pixel-art inspired April Fools canvas where Reddit users come together to create art and defend their claimed territory) and some MMORPG communities (Personally, I've seen a lot of kindness in The Lord of the Rings Online & in Furcadia back in its heyday). Of course, there are some terrible places, too, like the ever-infamous COD lobby, full of bravado, insults & swearing. I haven't even played COD but I know of that reputation, so it definitely speaks for itself.
In my point of view courtesy nowadays is being treated as a necessity to talking someone for their needs rather than as in good manners and that includes politeness.