> Aren't monopolies is what we end up by default if have no regulation at all?

No. Monopolies are only inevitable if the goods aren't elastic, if there is a large cost of entry into the market, or if its a market you can create a moat that is unsurmountable.

Many markets don't have that even with 0 regulation, but might have second order problems like firms creating unsafe products for example.

But in general regulations almost always even unindentedly raise the cost to enter the market. If you make a new regulation that food needs to be safe, then the company needs to pay a safety inspection that a small home-made recipe might not be able to afford (to give a simple example).

At the same time, we now have uber large corporations due to non elastic parts of supply chain (like land) or moats that are insurmountable (like access to US capital). In which case, the FCC should break up monopolies as the current market is not catering to end users and consumers but to owners, which is why the Stock market has been in a never ending bull run.