But, to your point regarding prorated comp: I've been a salaried employee at a company like Google. I've also been a board member.
First, like many senior tech employees my total comp market rate is in the seven figures. A pro-rated 1.2M stipend would be appropriate to compensate me for my time. The average board retainer for less profitable companies is closer to $30k/yr. These are not entry level positions and the retainers are shockingly low in the vast majority of cases. (In my case, I'm on the board of a non-profit and I actually pay them)
Second, I think you are underestimating how little some salaried workers actually work. I think if you try you can find more than a few Google employees who only work one month a year ;) Conversely: I work far harder in my role as a board member than I used to in my salaried role. It's different for everyone of course, but I assure you no one is seeking out board seat retainers as a way to get rich. It's just not worth it.