They can’t make them cheap enough, or legal enough.
AirPods are the first true wireless headphones, Apple invested a ton of resources engineering the shit out of fundamental wireless protocols and protected them with loads of patents.
Also initial batches of AirPods ran into availability problems, partly because the production plan wasn’t finalized until just before the official announcement, also because technical difficulties mass producing them.
The product material costs aren’t actually high, but very low yield and low chances of reworking drove the actual production costs up.
Other manufacturers can’t just use what Apple patented, they are forced to invent alternative approaches, or buy other solutions.
No solutions existed until recently, then they still have to solve the margins problem. They can’t sell them at anywhere near the real AirPods and they have to maintain profitability, which basically drove them out of existence.
However this is changing recently. There are valid true wireless earbuds solutions in the market now, AirPods lookalikes are showing up.