One problem is that there simply arn't anyone creating baseband chips which are "open". The closest thing is osmocom-bb, avaalable only for one (very old) chips

One problem is that there simply arn't anyone creating baseband chips which are "open". The closest thing is osmocom-bb, avaalable only for one (very old) chipset(which they had to reverse-engineered, with a big thanks to leaked documents on dodgy chinese websites). Another other issue is the carriers want the baseband chisets and protocol stacks certified before allowing it on their network.