I'm an software engineer myself there's 2 bottlenecks with any type of development and it's time and money. Yeah they could spend time/money making new skins, or they could make new champions which also generate revenue and have the potential to get new people interested in the game. It's easy to keep customers, it's harder to get new ones.
There are also other things like bug fixes, champion updates, balancing the game, story and narrative, philanthropic efforts , and, more specifically, the professional aspect, that take up LOTS of time and money.
As far as specific skins themselves getting produced, there's two I can think of; ideation and execution. It's kinda hard to come up with skins, not just the general idea, but how every specific of the champion. And once the idea is done, you have to put into the game in a way that has to be recognizable and distinguishable from a lot of other things in game in a millisecond. This takes a lot of time. I would say if a team worked on a skin start to finish would take 3 months at the least. And that's excluding the official QA process, redesigns, and post-release changes for accessibility.
A coworker worked at Riot and he said they are pretty intense in the amount of effort they put into their work, so I don't they they want to release low quality skins just to release skins, despite what people on the boards think. Hell, they made entire narratives and gameplay from just a could skins.
Edit: Sorry this is kinda long and may have been ranting but I hope it helps.