A few could paint a page a day, but most averaged two a week. Give a 60 page story to a painter and you have to wait seven months for completion. Give the same story to a reliable penciller and you can cut that time in half. Sure, the art still has to be inked and coloured, but this can be done while the penciller is still at work. Few editors have the luxury of time which painters need to finish their masterpieces.
Money is another issue.
Painting a comic strip takes longer, so the artist understandably expects a higher page rate for their extra effort. But unless the painted comic sells more than it would as a conventionally produced title, the extra page rate only means less profit for the publisher. Sure, painted comics are perceived to have higher prestige. But just as fine words butter no parsnips, so prestige adds little to a corporate balance sheet. Wannabe artists saw the pages of 2000 AD filled with fully painted art and deduced the only way to break through was by becoming painters themselves.