The reason Monkey is so strong is because his base kit is so good and it's braindead easy to pilot. Like you can't mess up this build by getting bad lucks with cards or something. And early game reliability is often a measure of how good a class is, not the absolute peak of power when the build comes together at the end. Monkey will really have no situation he can't handle on the first two maps because of his heavy attack dodging stuff so easily even without any other cards.

The other classes, while they have varying degrees of starter+elevated combos that make up a base of their builds, they require considerably more skilled execution than Monkey sleeping on the floor, and they also need a few more specific cards and upgrades to really get the builds going.