This is predicted by the hypothesis that the origin of intelligent life requires the occurrence of several hard steps, each of which take at least several billion years (but with no upper bound) in expectation.

If that hypothesis holds, then conditional on all the steps having completed on a given planet, they will have done so roughly equally spaced in the time allotted, regardless of their relative difficulty.

So, for example, if the window on Earth is 5 billion years, and there are four hard steps to complete, then, conditional on all four steps finishing, we expect each step to have taken about 1 billion years, give or take, with about 1 billion years left to go.

(See the Technical Appendix to Robin Hanson's Great Filter essay: http://mason.gmu.edu/~rhanson/greatfilter.html#appendix, or his more detailed discussion of this particular point: http://mason.gmu.edu/~rhanson/HARDSTEP.PS)

Why do we think the origin of intelligent life requires a few hard steps and not just a continuous progression or perhaps one hard step and otherwise a steady progression?

I think the hard steps are sharp divisions that had a long time in the status quo, then a sudden random development. Single-cell to multi-cellular. Development of nerve tissue that can collect information, make decisions, and influence the organism. Centralization of a bundle of nerves into a decision making center. Then you get to higher order steps that probably aren't as complicated, such as communications and organization, then all the steps that humans went through that took a while, without having major biological changes that enabled them.

I think any step which occurred independently on multiple occasions is probably not a hard step. The origin of multicellular life from unicellular life occurred on at least three separate occasions independently.

However, I think at the very least that the origin of the first cells, as well as perhaps the transition from Archaean/Prokaryotic cells to our symbiotic Eukaryotic ancestor may qualify.

We don’t necessarily think that from first principles. But the hypothesis does make certain predictions which are consistent with what we see.