It is, but it also takes time to reach those velocities. Every orbital rocket that I am aware of has a period called MaxQ, where it has to throttle back to avoi

It is, but it also takes time to reach those velocities. Every orbital rocket that I am aware of has a period called MaxQ, where it has to throttle back to avoid exceeding air pressure limits. But it only occurs in lower dense atmosphere and usually ends in a few seconds. So balloon drop from high altitude avoids having MaxQ issues at all, so no throttling required (which could also make rocket engines simpler). But all the other drawbacks make air drop only useful for the tiniest of orbital payloads.