I worked at a cellular base-station company back in the oughts. The technical reason back then was that phones hopping between towers so rapidly caused fake capacity spikes. It took just about long enough to negotiate a connection to a tower and suddenly boom, phone has moved on to the next tower, leaving a binch of timeouts on the old tower.

Hundreds of phones doing this for each plane caused problems. Now the negotiation is faster and tower capacity is much higher, so no big deal.