1) It would generally be a poor investment choice for an employee to invest their savings in a startup that they were actively working for. Startup employees are already over-invested in their company from a diversification perspective.
2) The majority of startup employees are unlikely to have the cash on hand to make that kind of investment.
Because of #1 and #2 this isn't something that most employees would care about so it's not part of any standard compensation package. It's possible that an employee could negotiate for such a provision though.
The fact that you would want a pro rata right to work at a startup does make you fairly unusual and I agree goes to explain why you have chosen other career options.
If I was going to work at a company for X% ownership, I'd sure expect to have the right to invest my own money to preserve my stake in a funding round and avoid dilution. Many employees might not exercise this privilege, since it would require putting (potentially a lot of) cash back into the company, but why aren't they given the choice? I also might expect to receive the same liquidation preference on any shares purchased in cash this way.
I'm not sure I could be comfortable working for a startup without pro rata rights and relatively full knowledge of the cap table and preferences. That's probably why I'm not working at a startup.