The market for developer talent and designer talent are visibly different. Different in terms of supply, compensation (as a factor of supply), and risk (due to the compensation!). Risk here is you mostly take the word of the developer they're competent. We come up with elaborate interview processes for this (if you have an idea for a better process... ). For a designer, a portfolio clearly demonstrates they're competent. This makes outsourcing and freelancing more viable, increasing supply, and driving down the overall costs, in my opinion.
I won't touch on actual skill / investment in becoming a good designer vs. developer except to say I think there's differences there too, at least outside of the tail.
I would circle back to my second point, that even if you have an excellent designer, someone still needs to implement it and not make it janky. I think it explains, as a whole, why people gravitate towards engineering co-founders if they don't have the skillset.
> Consider that the #1 market cap YC company of all time - AirBnB - was started by 2 founders with a background in design.
I don't think Airbnb won on design or would consider design its moat. Designers can also be excellent leaders and executives.