This has been one of the biggest criticisms of Arena from its very start. It is supposed to be the "competitive mode", yet you are very likely to meet opponents far away from your skill level (and you can't really avoid it even with the filters available). Because everyone starts at the same tier and then later the tiers don't even discriminate that much between skill levels, you are more prone to random opponents than you would be if you used "normal mode" and filtered properly by Elo levels. This is a complaint that comes back again and again from players in all Elo levels.
Here is some BGA lore: Soon after the Arena system was introduced, they tried to fix this problem by having an Arena schedule so that Arena games would start only every 5 or 10 minutes at fixed times. That way players could accumulate between the slots in the "starting box" so that then enough players are available at every Elo level when the games start. I loved it - you always got opponents close to your own Elo level. It really felt like a competitive mode, however, turns out most people hate to wait for their games to start, even if they get much better fitting opponents for that. BGA received one of their biggest shit storms and Arena traffic went down overall, eventually BGA dropped this system after only 24 hours.
I always hoped that BGA would find some middle ground system to solve the trade-off between low wait times and getting better matches, but they never touched the Arena system again, only very small tweaks here and there.
Also, I have to say, most people seem to care not that much (much less than I thought) to play opponents not matching their skill levels. For me, having a skill-matched opponent is a big part of my enjoyment, particularly when I want to play competitively. but it seems many people are different in that aspect.
