A link to imgur with some pictures (taken by me, forgive the quality) that may be useful: https://imgur.com/a/0ctoD

First off, I'm not going to dismiss your claim by going off about the brilliance and vitality of ancient painting. Just look at the first two picture in the album, a Menander from Pompeii and an Apollo from Nero's palace: Menander is kind of just ok, and Apollo just looks ridiculous. So I know exactly what you mean by "crude."

But here's the issue. In sculpture, we have many truly excellent works of antiquity, or at least very good copies of them. Marble and bronze are quite durable, after all. Painting is much more perishable, and we have much less of it, and most of what we have isn't of the first rate, or second rate, or in most cases the third rate. That Menander and Apollo are just wall decorations.

There are, however, some remaining paintings that suggest the level that the real Greek and Roman masters had reached, and in my opinion it probably equalled anything from the early Renaissance.

The third picture is from the Villa of the Mysteries. It's hard to get good pictures of because of how they have it blocked off, but you can find many online, and it's just beautiful, and the detail is amazing, and the characters look alive and moving. And this is a painting that is not mentioned at all in literary sources and was apparently not famous, but rather, just a fresco that was put up in some (insanely rich) person's house.

The fourth is an Aphrodite. It's not as good as in the Villa of the Mysteries, but it's in a much smaller and less wealthy house, and it's still quite attractive. But here's the important thing about it: it's probably a copy of a work by Apelles, the most famous Greek painter of antiquity, who was universally regarded as a true master and of whose work we have absolutely nothing. But one of his paintings was an Aphrodite like this, and it was apparently famous enough for people in relatively modest houses in unimportant provincial towns to have it painted to decorate their gardens. And this copy in a modest house in an unimportant provincial town gives us, I think, some inkling of what the original must have been.

One final example I don't have a personal picture of, so I'll link to Wikipedia: https://upload.wikimedia.org/wikipedia/commons/c/c3/Battle_of_Issus_mosaic_-Museo_Archeologico_Nazionale-_Naples_BW.jpg

This is the famous Alexander mosaic. It looks a little crude, because it is after all a mosaic, but even in this form it certainly far surpasses any medieval painting I've ever seen. Look at the details: the man grabbing Alexander's spear and twisting aside, and the famous dying Persian's reflection in a shield. And remember, this is a mosaic. And the thing is, this mosaic is actually a copy of a painting by, you guessed it, Apelles (probably). If a mosaic copy looks like this, again, the original must have been glorious.

So, Greek and Roman art as they survive do indeed look rather crude. But not much of it survives, and what survives isn't the best or usually even the second-best. You wouldn't think much of our art either if all you had to go on was the paintings people hang in hotel rooms or posters hung in dorm rooms. But we know that there were painters who were regarded as masters, and there are enough hints among surviving copies of their work (or copies of copies) to suggest that their works must have been magnificent.