I'm in Germany, so I quickly googled for some stats on Amazon's market share. [1] In Q2 2019, their biggest market share is in electronics at 24%, with toys/sports at 18% and books at 15%. Everything else is 6-10%.
I understand your concern, but the reason that anti-trust legislation is not addressing this is that there is frankly no monopoly here.
[1] https://de.statista.com/infografik/16018/marktanteile-von-am...