Not going to speak for the modes team but as the designer who owns champion select I can give a bit of context on why I think this would be challenging.
The intent of the % based buffs is to take champions who are primarily designed for other modes and bring them into line based on the conditions they’ll encounter in ARAM: To make them more fair.
They aren’t meant to be a major factor in your decision making process. If you displayed them on the champ select screen we’d probably see players biasing towards champions that have “buffs” or increased damage and reduced damage taken. After all, they’re stronger right?
But that would be a mistake! Champions with lower %s have those because they have other built-in advantages on ARAM like consistent poke or very high range. You could easily assemble a team comp with good %s that gets thrashed by a team with bad %s.
TL;DR: We don’t show them because they could be misleading.