Short answer: they do lol.
Longer answer, geared somewhat towards this:
Generally speaking the situation that you're describing in something of a minority case- if you look at almost any comic that has been runing for a decently long time (say several months or more) and you look at one of the earliest episodes and then at one of the latest episodes you will almost certainly see some art improvement. I can't recall very many cases where that didn't happen, except perhaps if the artist was already starting at a very high skill level and thus improved in smaller or more subtle ways over a long period of time- there is always room for growth, but it's less dramatic of a change the better you get xD
THAT SAID I do think what you might be seeing are some artists that work for a very long time and just don't show that much improvement? This is something that I like to prattle on about a decent amount, but generally speaking I think that is often caused by an artist either staying in their comfort zone too much, or some of the other numerous reasons listed above about the comic-making process, such as time constraints forcing them to rush & etc. (although even then... unless they're on contract, the artist can always ease the schedule if it's making them rush too much
)
Basically I think that an artist improves faster the more that they try things that are slightly above their skill level or outside their comfort zone rather than by just sitting in their creative bubble and trying to improve incrementally over time. It's like the classic "hide the hands behind the character" schtick- you see a lot of younger artists who haven't quite figured out hands either hiding them behind objects or just putting them out of the scene or etc. rather than risk drawing a bad hand and ruining the rest of the picture. However by doing that they're just stunting their growth by avoid the problem rather than facing it head on and trying to get better at it.
A common comic example could be something like backgrounds- they're hard and time consuming so a lot of people aren't fond of them. This can sometimes lead to a bunch of talking heads in space and then when the time comes that a background is necessary, it looks... not great, because they haven't been practicing how to do them as much as they could/should.
The same goes for using boring panel compositions, or camera angles, or perspective types, or anatomy, or anything really. I think if you see an artist that's just not improving that much over time, they're probably either just not doing much work over that span of time (for example, my art from 2012-2017 all looked more or less the same because I took a pseudo art break while in was in college between those years) or they are doing a lot of art but just not pushing their horizons enough. That's not a bad thing by default... as, again, a lot of other people have listed out above. There are a number of reasons why improvement isn't and shouldn't always be the primary concern while working on a comic. But yeah.