In my ~20 years of mostly startups and small businesses I have seen waterfall creep in at almost every company. You start by being small-a agile because there is no process. Then something happens – a few big fires ship, or someone misses a requirement, or an exec gets grumpy about feeling a loss of control, or internal users start treating it as just-a-job and want official training for every little thing, or you get engineers who just want to code – and your small-a agile process turns into waterfall.
Ah but we’re a startup, we can’t do waterfall! So you do a series of tiny waterfalls and call them Agile. Everything takes eons of planning, the management class feels super busy, the engineers become disempowered and widget factory like, and all the plans are wrong by the time you implement.
You start getting lots of “we’ll figure out the details then engineers just need to implement, it’ll be easier that way”. It is not easier that way.