Which, while making the DM's bookkeeping easy, doesn't incentivize anything other than showing up; and doesn't even incentivize that if it's character-in-play sessions being counted rather than player-at-table sessions. It doesn't incentivize adventuring, it doesn't incentivize risk-taking*, and it doesn't incentivize advancing the story.
Taken to a silly (but very plausible) extreme: the characters could advance in level by spending several sessions just sitting around at the farm telling war stories as freeform roleplay.
* - in fact, it actively and strongly disincentivizes risk-taking: if you're going to gain levels at the same rate anyway, why stick your neck out?