On some level that makes a lot of sense. If you're several years from IPO (or expected acquisition), 10/20/30/40 won't do any harm to people who stick around until the stock has value, and it helps keep short-turnover employees from eating away at the total stock.
Flat vesting sort of implies that four 1-year tenures are just as desirable as one 4-year tenure. And if people can get a better cash-compensation raise by switching jobs, then staying 4 years is probably less profitable than staying 2-3. (Of course, companies could also cure that by making internal raises competitive with job-transfer raises...)
Although I do find .../30/40 a bit stranger. If somebody leaves at 3 years, you save 15% of their stock compared to flat vesting. But if the extra stock gets them to stick around, they're obviously going to leave after 4 when their effective compensation plummets. It's not obvious to me that pushing employees with no long-term intent to stick around and claim more stock is a winning move.