HA doesn’t require 4/4/32, that’s just the hardware the HA people sell. (which, given that your phone may be 8/16/128, is hardly “robust”). Generally, the Home Assistant crowd kind of target an audience that’s probably already running some kind of home server, NAS, or router, and HA can probably be installed on that device.
Theoretically, there’s no reason the HA server couldn’t be installed on your phone, except then your smart home functions would only work while your phone is in the house and not sleeping. Kind of defeats the point of a lot of it, unless you’re just thinking of smart home like “remote control for everything.” Regardless, much smaller niche for an already-small market, and apparently not a priority for the dev team.