Because for example OPNsense recommeds at least 8GB of RAM, though the bare minimum is 2GB.
And even purpose-built enterprise routers are certainly in similar ballpark. BGP will eat that.
I don't think I've seen one of those dumb ones with nearly that much RAM. Usually they're like 16MB/32MB. Guess I'll have to check.
My Mikrotik router at home (entry level enterprise gear) has 512MB RAM, and it can do most of what those bigger routers can, just a lot fewer connections.