It's definitely more than that in software. RSU cliffs are a real thing, so in a stable market or growing market, it's not even that the new company needs to offer you substantially more than you were making at the 3 year mark, it's that your effective pay might drop by as much as half at the 4 year mark.

OPs question has some merit, because it's a bit infuriating that they'll replace you with someone who has no domain knowledge and give them a 4 year RSU grant, but they won't just refresh your RSU grant with grants that are anywhere near as good as the initial grant. Hell, I work with people who took other jobs and then came back to get another big grant.

At that point, it's policy, the company (for whatever weird reason) wants the turnover and fresh blood. Personally I think there's just a lot of under the radar ageism in software engineering, and this facilitates it, but who knows. It's not like that everywhere, but a good chunk of places I've bounced around at had 4 year cliffs.