Back in the 20th Century, many desktop computer systems played a simple beep at startup to indicate that Power On Self Test (POST) had completed. Startup key combinations could not be recognized by the system until POST was done. When Mac came along, Apple thought it would be fun to replace that annoying beep with a more pleasing sound.
POST used to take a fair bit of time (10-20 seconds or more), but today's computing devices finish POST in a fraction of a second. There's no need to "wait for it, wait for it..." before pressing key combinations, so Apple eliminated the startup chime from Mac a few years ago. Apple returning the startup sound to Mac is purely about nostalgia and marketing (announcing to people in the area that there's a Mac present).
There's no functional need for a startup chime on iOS - there are no key combinations or gestures that can be invoked at startup.
All a startup sound or animation (Apple logo with loading bar) is useful for is to tell the user that something is actually happening. So sure, if the iPad can play a startup animation, why not a sound to go with it? One argument against it is that maybe people would prefer silence when starting/restarting an iOS device, since it's often done in public, rather than the relative privacy of a home or office.
But would Apple do it? Not if they want to maintain product differentiation between Mac and iPad.