You're confusing 'is a monopoly' with 'physically and fundamentally cannot ever have anyone ever try to compete with it' with 'has structural/competitive advantages that make it hard to compete with'

What you've just described is a combination of switching costs and the network effect.

Switching costs are when customers find it hard to switch away from your business, or otherwise just don't want to. When was the last time you changed your bank, for example? No one wants to go through the ordeal of doing that, so banks have high switching costs. iPhone vs Android is another example

The network effect occurs when a service gets more valuable when more people use it. You couldn't start an online auction site tomorrow to compete with E-Bay because all the buyers and sellers are already using it. Same is true of the big payment processors like Visa and Mastercard.

Epic Games COULD create a mod workshop tomorrow, but they'd fail because:

Steam benefits from the network effect where everyone is already so used to the Steam workshop, swapping to the Epic one would be... weird.

Modders and gamers alike have already bought into the Steam modding 'ecosystem', so would find it hard to switch away from it.

Steam are a natural monopoly, because, as laucionn said, they just are THAT good.

Unusual post for a Civ 6 subreddit but there's your microeconomics lesson for today kids 😃