Shield TV has 3gb and runs great. The bottleneck for tv boxes is usually the processor or the network.

Not sure what your use case is but tv boxes usually only run a few apps and don't multitask much so manufacturers aren't making them with 4gb ram. Even the new super expensive JBL sound bar has 2gb RAM.

Edit: Shield has 3gb not 2

You will be very hard pressed to find one with more ram as they simply don't need it.

Android utilises ram differently to say windows, and as such ram needs are much lower, unless there's a specific program you want that needs more.
They are designed to basically always use whatever ram is available, so 90% ram useage is perfectly good.

The OnePlus 7 is definitely the exception, not the rule, as that's a massive amount of ram at 12gb. Considering it's got a 48MP camera however, I can perhaps think of a few reasons!

Basically, 90% of android tv boxes are, well, pretty basic, at least all the china ones anyway. And they simply don't need more ram as the ram isn't the limiting factor.

Useless as tits on a boar!!

Android memory management is pretty good. It closes apps down in the background when it needs more ram and a TV box doesn't have an app that takes more than 1 gig of ram. Games might take more ram but just buy an Xbox One or PS4 if you want to game.

Why?

Why greater than 4GB ?

Since Android Oreo, which contains a bunch of optimisations and much better garbage collection routines and memory management there really is no need to even go greater than 2GB.

A powerful GPU and high quality fast RAM are far more important on any Android TV media player, that is mostly used for video streaming.

High quality RAM is relatively expensive too, something Android TV vendors want to minimise due to cost.