Why aren't all champions unlocked for custom matches?

So, I've been saving up my IP for a while now and i have enough for a 6300 IP champion, but I don't know which champion to buy because I haven't tried them out. I don't want to spend my IP on a champion and end up not liking them, then either using a refund request or keeping them until I save up again. If this is implemented in only custom matches, it will have little to no effect on match made games. It is a chance for newer players to try out champions that they missed on free rotation and won't be back for a long time.