Why would they? Draft after schools. Still need to develop the boys in the academy.
On the one hand, hurr-durr, yes. I is not clever. On the other - there's still a lot of training and talent identification going on at school level which would be at risk.
Also - a post school draft seems too early. Most players seem to enter academies a year or so at least after school. Post-university would be too late though and rearranging all the academy players halfway through seems quite a lot of disruption. Its not like Connacht aren't already bringing in academy players from Leinster and Munster already.