Uh… Nintendo DID hire professional players to help with the development of Smash Ultimate. Their names are Tomoyasu Yamakawa and Ryuto Hayashi, known in the Smash community as Earth and Ranai respectively.

it is there to weaken the effect of juggling with a single attack (such as the forward a spam ) and to encourage new players to try and use their full arsenal as far as i can see
now i don't even think it follows that this rule exists because of insufficient testing i don't see the logic there but i must also remind you SAKURAI is known for adding weird rules because he's that kind of guy see tripping.