Price?

NPUs are generally a different thing to the common forwarding logic in switch ASICs / SoCs. NPUs are programmable logic that are aimed at processing data as it streams through, applying transforms to the data (possibly with the aid of other fixed blocks of logic, e.g. cryptographic functions). The switching ASICs and the switching blocks in SoCs in these kinds of AP/routers are generally fixed-function pipelines, configured via tables (mostly). I.e. you setup lookup tables (e.g. enable or disable whatever features, and then create entries in the tables that control those features) - and the fixed-function pipeline applies its logic to packets, guided by lookups in tables. The logic is not generally programmable - it's not an NPU.

I can't find programming docs for the PSE in the MediaTek engine, but OpenWRT patches I can find for stuff like "Wireless Ethernet Dispatch (offload of wifi packets to PSE) look to be the typical programming of tables for fixed-function pipeline.

Generally, where you see NPUs, you'll still see a fixed-function packet processing pipeline. Cause the latter handles the common case much faster and with lower latency than NPUs. The NPUs will be there to accelerate the "slow path" - allowing uncommon case packets to be processed faster than entirely on CPU.