>I'm just looking for an indoor air quality monitor for temperature, humidity, co2 and ideally pm2.5 and pressure.

That's because it's $$$$. CO2 is hard to measure (it's inert!), and only recently advances in NDIR allow us to measure it directly. Those are still $50, and that's just component cost.

PM2.5 is also around $50 for decent parts.

Another $10 for T/H/P. Another $20 for cheapest LCD you can find.

Ok, so we are up to $130, but now you gotta pay for manufacturing ($$$), PCB, hardware development, calibration, testing, mechanical design... Probably $200-$300 total depending on quantity.

So, let's say you can make the whole device for $250 just in costs. Now you gotta add software (BT\Wifi code is not fun!), shipping, overhead, profits. So, you gotta sell this thing for $350+.

Would you buy one for $350?