The original owners have since sold off the company to BBK Electronics in China and now China manufactures them. Now they sometimes share designs with other Chinese phones like Oppo, but more importantly OxygenOS has changed from a mostly stock Android rom to basically the US version of the Chinese ColorOS. So not only has the hardware been copied and pasted, but so has the hardware and people just don’t trust that.
Now the old CEO of OnePlus ended up ditching the company to form his own startup Nothing. There currently exists the Nothing Phone 1 and 2 and I’ve kinda been eyeing them in case they make a super interesting product one year.