I mean, to me it’s often rubbish vs. rubbish, but I think there are a few issues intersecting here:
Purely representational art as covers tends to be seen as tacky. Not without reason. It’s also very visually busy, in many cases, and can make books hard to distinguish from each other at a glance if they’re new to you. However I would say purely abstract covers aren’t as common as you’re suggesting - a lot of series use some representational art and some more abstract stuff.
Audiences are much more annoyed by inaccurate representational art now. Half the books which had representational covers on them back in the 1990s and earlier had drastically inaccurate imagery. In the pre-internet era, that was rarely an issue except in annoying some authors. Post-internet, though, it’s clear it annoys fans too (it always did, but you couldn’t find people to agree with it on). It’s also a liability, potentially. Often the artists of representational stuff would get the race or other significant aspects of a character wrong. In the earlier 2000s this happened a bunch of times and it was pretty damn embarrassing for all involved (virtually no Asian-looking character was represented as anything but white, for example). Or they’ll make a very dark-skinned character relatively light-skinned, or a mid-tone character pale as ice. This was an issue with a lot of Wheel of Time stuff - Lan is basically Asian-looking, and apart from Rand, most of the main characters are darker-skinned, dark-haired and dark eyed (they characters even comment on this - first city they go to they marvel at how much paler a lot of people are than them). Yet virtually every representation has them pale as fuck, and you’re lucky if they even stay brunettes, let alone dark brown-haired (fan art rarely gets this wrong, I note). Their skin tones and so on were a conscious thing that tied into the backstory of the books too (the deep backstory even). Yet most of the covers have them just looking like a bunch of white kids, who maybe went to the beach, but are clearly white (which Jordan himself said wasn’t right).
Art is more expensive than it was, relative to its importance, especially outside of “collectors edition”-type deals. Yeah, there are more artists now, but there are also more outlets for art - a lot of people can make a lot more money doing art for Magic The Gathering (each card effectively a cover) or concept art for video games and films than they could doing book covers. And it matters less to the sales of a book what the cover is, so long as it’s not awful, because people browse and buy books differently. Only when you are selling special editions and the like can the importance jump up. For example, there’s a special edition of Robin Hobb’s Assassin trilogy (a great series) which has large, painted colour plate artwork of characters and scenes.
Re: Wheel of Time - you show the old covers and the slightly older covers, but not actually the newest ones, which go back to a mixture of representational and abstract, and can be seen here:
Bottom row are the new. I dunno why they arranged them in that confusing order (middle = oldest, top = slightly older, bottom = new). Unfortunately the new ones continue the trend of making the characters all look “white”, but like, less white, and their hair colours are more accurate.
Looking at my Kindle and Audible (I don’t buy RL books anymore), I’d say that it’s an awful lot of books still have some representational art, either literal or semi-abstract (or whatever you want to call it - man, for all my art education I sure am rusty on terms today), and the books which just some sort of “symbol” or the like on the cover are in the minority, as are those with highly abstract images. Gideon the Ninth, my favourite book of recent years, has a representational cover:
Note that the representational art is extremely accurate in a way almost never seen in the 1980s and 1990s. If you pick through points on the character, pretty much all of them are correct, though people might have questions about the rapier (she normally uses a greatsword), but she does at least wield one in some of the book (the titular Gideon is pictured). And that accuracy wasn’t an idle decision. If you don’t do that now, people notice, and people get mad.
(The author gives an amusing breakdown of the artwork here: https://www.tor.com/2019/09/13/from-skulls-to-swords-dissecting-the-cover-for-gideon-the-ninth/)
In the end, representational art isn’t dead, but it now has to be spot-on accurate, and is harder to justify in terms of being necessary to sell a book, as well as having to be carefully pitched to avoid being tacky/twee (unless that’s the goal). The “Harry Potter” era, i.e. late 1990s through about 2008-ish was the “low point” for representational art. Bookshops, even before Potter, were starting desperately to try and make fantasy seem “cool” and “not just for nerds”, and the sort of twee 1980s-ish representational art a lot of them had (looking like either a D&D page or a prog-rock album cover or both) was not helping with that. So they overreacted and relatively few books from that era have representational covers unless the series started before then, but things have changed since.