> Is it just a perpetual motion machine of market sharing affording good marketing which then drives continued market share?
Worse. It's a market where most of the customers are unsophisticated but price sensitive, so they tend to prefer the provider with the lowest apparent price, and then the big providers compete on the basis of who can present the lowest apparent price through the use of dark patters, misleading claims, bait and switch tactics and hidden fees.
Example: GoDaddy provides a "free" site builder but if you use it the resulting site can't easily be extricated from their service and now you're locked in if you don't want to recreate your site. Meanwhile the price you were quoted for various services was an onboarding price and now that you've sunk a lot of time creating and improving the site you can't move, the price is going up.
This is, incidentally, a major reason WordPress is so popular despite being fairly miserable. It makes it easy for unsophisticated users to get started and your site isn't tied to a particular host.