Say you're a vendor producing widgets for $10 each, costing $9 each to produce. 80% of your buyers prefer the buying and service experience of Amazon, for all the reasons you describe. They have no brand loyalty to you: they go to Amazon, type "widget" in the search box, and buy the first result with 4+ stars costing roughly $10.
Amazon decides out of the blue that they want $1 per sale, and what's more, they will drop you from their platform if you offer a different price on Amazon and other storefronts (be it another giant like Walmart, or your own low-traffic e-commerce website). Because this $1 is the entirety of your profit margin, your only way to stay in business is to raise the price to $11, and because your widget competitors have been given the same deal, all widgets now cost $11, and when consumers are price-shopping, they're now conditioned to see $11 as the normal price for a widget. You could opt out of Amazon entirely, but that would limit you to 20% of revenue, which is also a dealbreaker. (You theoretically get a higher cut from that 20% at the new price-point, but you also suffer the deadweight loss from buyers who can't/won't transact at that price point.)
The standard libertarian retort would be that the seller consented to their contract with Amazon, while no citizen or market actor ever signed some imaginary "social contract" with state regulators, and it's a legitimate point. But saying that a vendor whose livelihood is dependent on a presence on Amazon.com (whose revenue rivals the GDP of many nations [1]) "chose" that relationship is like saying that each citizen "chooses" the laws of the nation-state they live in, because they could have moved elsewhere. Yes, technically there is an exit clause; but in both cases the table is tilted such that exiting is deeply against the interests of each actor, thus each actor is incentivized to perpetuate the asymmetry in negotiating power.
Market competition is a great thing when it works, and many times it does. But oft it's forgot that the whole point of a competition is to win. And whether we're talking political power or economic power, concentrations of power tend to snowball and entrench.
[0] https://en.wikipedia.org/wiki/Market_power
[1] https://www.axios.com/big-techs-power-in-4-numbers-de8a5bc3-...