Light wavelengths are in the order of nanometers. Radio wavelengths (were interferometry is "solved") are in the order of kilometers to centimeters. While it is possible to build structures that are unmoving in the order of centimeters, it is hard to impossible to do that in the order of nanometers. That doesn't make direct optical interferometry impossible, just quite a bit harder, see e.g. LIGO.

And for doing timing-based interferometry over a distance, the needed timing precision limits your resolution. 1km/c is in the order of 10^-6s (µs), "easy", 1cm/c is in the order of 10^-11s (ps) "possible", 1nm/c is at 10^-18s (as) "quite impossible".

And for the layman, a factor of 10^-6, or 6 orders of magnitude in time are like "a second to two weeks" and "two weeks to far back into the stone age".

What do you mean by solved ? optical interferometry exists since the 70s, and is used a lot. The VLT, Keck observatory, CHARA, LBT, NPOI. It's not has big as classical observation because there are a lot of disadvantages, but it exists