Apparently they wanted the Apollo style space-age interfaces with all the knobs and switches. Just, they didn't have the budget, and paint was cheaper.
https://arstechnica.com/gadgets/2016/09/how-star-trek-artist...
> According to Michael Okuda, original Star Trek art director Matt Jefferies had practically no budget. "He had to invent an inexpensive, but believable solution," he told Ars. "The spacecraft of the day, such as the Gemini capsules, were jammed full of toggle switches and gauges. If he had had the money to buy those things, the Enterprise would have looked a lot like that."
> What could be simpler to make than a flat surface with no knobs, buttons, switches, or other details? Okuda designed a user interface dominated large type and sweeping, curved rectangles. The style was first employed in Star Trek IV: The Voyage Home for the Enterprise-A, and came to be referred to as "okudagrams." The graphics could be created on transparent colored sheets very cheaply, though as ST:TNG progressed, control panels increasingly used video panels or added post-production animations.
In the 32nd century, tho, they use programmable matter because people like tactile feedback, it turns out.
(Aside, in the novels they came up with a reason for this: During the Earth-Romulan war they had to go partially low-tech to avoid the Romulan telecapture technology, a technology they'd just developed to remotely hack into and control enemy starships. Apparently the design kept for ~100 years up through TOS when they finally learned more about the Romulans.)