There are actually two major reasons why I don't leverage Monarchy.
The major one is I know when I revolt out of Monarchy, I'm going to have major because I don't have the to replace the provided by Monarchy.
The second reason is because I usually don't have enough workers, so I build maybe 2 or 3 improvements around a city before moving on the next one, especially how I favour early rushes a lot.
Are we still talking about Civ IV? Civ III perhaps?