Replacing the knob is a safe operation that cannot leak water. I could do that on all radiators in a flat without ever running the risk of causing a leak. Replacing the valve would require opening the water circuit, thus you'd first need to turn of pressure, which may or may not be possible (think: apartment building). Replacing the valve may also not be allowed for people that rent a flat.
After all, if you're selling a £150 smart home system, but 80% of buyers need a £150 professional installation, the cost will deter a lot of customers; and for those customers who value your system at £300, you're missing out on capturing a lot of that value.
De/re-pressurising your central heating system and physically replacing all the valves is a task that's going to require specialist tools and probably a plumber. It's also pointless, as there's basically only one standard, so you'd have nothing to replace them with. Replacing the knobs on top is a case of watching a youtube video and spending two minutes with a screwdriver - which is the intent of the design.
What you're saying is broadly comparable to "why would you buy smart lightbulbs when you can just replace the entire light fitting and all the switches in your home?" - except I'd argue that's actually easier for the layman than messing with radiators.