Would think it's trivial to make those from cellulose or something, but I guess it's always about money. Most "biodegradable" plastics are just turned into smaller pieces of plastic by UV.
In the case of BBs they need to withstand a fair bit of force, so I don't think it's about money. The biodegradable ones (PLA) on the market I've tried perform less well than their non-biodegradable alternatives and can shatter on impact, cause jams, etc.
I doubt you could make a hard enough cellulose pellet, maybe it's possible but costly, idk.
