Startups need cheap workers who will work long hours. And honestly, for every one Google (who legitimately needed genius level scientists), you have 10 WhatsApps (which are glorified simple CRUD apps).
Now how are you going to get a guy (who sort of knows what he's doing) to work 15 hours a day for pizza?
You've got to trick him "Change the world", " we're the next Google ", " you get to use any stack you want"
When your a 20 year old kid, with no responsibilities and a future ahead of you, this sounds perfect. When you're 30 (and all the more so 40), you go to work to 1. Feed your family 2. If you want to "change the world", you work for a non-profit.
And all the skills don't matter. Your knowledge how to save RAM? Useless. Just spin up another VM.
So for the CEO of a startup, a 30 year old is simply too expensive for what he needs - a cheep code monkey. All the excuses are just that.