The old PHP is the target for people's unedited ht.access files
Sonic can't swap in the new PHP at the old target location, because there are users who have already edited their ht.access files to point to wherever the new PHP is stored now. Or Sonic would have to leave 2 copies of the new PHP live.
Maybe there's other business stuff at Sonic with the same problem of an outdated pointer to an old version of PHP, and someone would have to chase down and fix all the pointers one at a time, and nobody wants to assign that tiresome chore to some clever staff member who would burn out on it.
Or some manager's reaction to any mention of PHP is SHHHHH! Be Quiet!