Ideas are cheap because you can have them lounging your chair doing nothing but thinking. No risk-taking involved. No years of hard work involved.
The reason Silicon Valley (rightfully) values execution so much more than ideas is because it takes guts (putting in own or other people's money) and sweat (hard work, focus, attention to detail) and a whole lot of other things to take an idea to product on the street. This is why most ideas never make it to product.
So compared to ideas, sweat and guts are quite rare, making ideas the cheap good. Easy as that.
There was a time where I would've agreed with you. These days I know better. Good ideas are as cheap as "execution" (sweat/guts) guarantees success.
Success is a perfect storm.
By way of analogy - I did a U-turn with my spare time about 5 months ago. Instead of coming home to write code, I came home and created a 1:10 scale replica of a Ford Bronco concept truck from 2004. People see photos and go wild, "OMG the effort put into that is phenomenal" and "wow I wish I could do that", "dude those are crazy skillz" and so on.
That truck is spectacular to others, I just see the mistakes in an otherwise ok build. Because I'm loving it, the process of creating, using my hands. Much like most (note, generalization) people that started unicorns didn't start out thinking "man I'm so going to take a cheap idea and succeed by doing hard labour and live on the sidewalk for the next 12 months".
It's a perfect storm. Good idea. Timing. Mental state that doesn't distinguish between "hard work" and "fun". Belief, but mostly in yourself, and being ok with failure.
The reason Silicon Valley (rightfully) values execution so much more than ideas is because it takes guts (putting in own or other people's money) and sweat (hard work, focus, attention to detail) and a whole lot of other things to take an idea to product on the street. This is why most ideas never make it to product.
So compared to ideas, sweat and guts are quite rare, making ideas the cheap good. Easy as that.