Rafa is so exceptional that he could obviously win nearly any tournament at any time. So can Djokovic. So could Federer in his best years. But each individual possesses different attributes and different preferences. Nadal has always preferred to play a "full" spring clay schedule consisting of MC, Barcelona, Madrid (previously Hamburg) and Rome. Rafa likes to get those reps on clay. He likes the build-up of the CC season. Sure, we can speculate that he would benefit from eliminating a warmup event or two from his schedule at this stage of his career, but the evidence doesn't really support doing so. Clearly, he would rather cut back in other parts of the year so that he can work into the CC season and play his typical schedule.
It was widely spoken about (by Moya, by Rafa himself and repeated by various media members) that Rafa needed Monte Carlo, Barcelona, Madrid and Rome to obtain the necessary reps and confidence in 2019. Nadal lost in the semis of those first three tournaments before winning Rome two years ago. Then he swept through RG as usual. I'm not sure what can be gleaned from the abbreviated fall 2020 clay court season.
This year appeared quite similar to 2019, except that Nadal had played very little tournament tennis in months. In fact, following the fall 2020 RG, Nadal played in three official tournaments between October 2020 and April 2021 (three tournaments in six months). He looked rusty and off in Monte Carlo and Barcelona, but found a way to eek out the Barcelona title over Tsitsipas. Perhaps he didn't need to play Madrid, but he had a week off after Barcelona and it's in Spain. As in 2019, it wasn't until Rome that he really obtained the necessary confidence. He didn't lose to Djokovic in the RG semis because he overplayed. He lost to Djokovic because Djokovic is equally exceptional and happened to play a bit better on more key moments later in the match.