What's wrong with people only buying champs that they think will be strong in that gamemode? I put significant consideration into my purchases and try not to get weaker characters; why is that a problem?
The problem is that it runs counter to League's account progression. In every other game mode, owning more champs is unarguably a good thing. It enables trades, allows for larger personal playable roster, and even provides a way to show how much a player has invested into their account. There is absolutely no downside to owning a champ in League, except in ARAM.
imo, there are no 'best' champs, games are dependent on skill
"Best champs" are hard to define, but "better champs" definitely exist. For instance, look at Bard or Rek'Sai. Both of them have significant parts of their power budget tied up in abilities that aren't particularly useful in ARAM (Bard E, Rek'Sai passive and ult active). On the other hand, Sona is designed to be strongest when she's around her team, and you're almost always with your team in ARAM. Player skill still factors in, but that doesn't mean you're not playing with a handicap.
there is still a significant chance that they will get a champion from the free rotation. If they don't want to play champs from the rotation, then they need to buy more champs and expand their champion pool.
If someone owns the 30 strongest champs, at worst they have roughly a 1/4 chance of getting a champ from the free rotation. Coincidentally, they'll also earn one reroll roughly every four games.
ARAM is for allowing people to try new champions (i.e. the ones in the free rotation), not forcing them to play such a huge variety of characters that there's no chance to actually develop skill for any of them.
That's up for personal interpretation. Some people like having everyone have to adapt to random unexpected champs.