Why is the Marie Curie Individual Fellowship only for 'experienced researchers'?
Funders are allowed to make their own rules. Typically they try to be as fair as possible, but there will always be some 'edge effects' where the rules have unintended consequences.

In the case of Marie Curie grants, there is an explicit intention to draw in people from across the entire EU (and beyond). As such, the rules need to accommodate the wide variety of different systems, norms and expectations found in different countries. The clearest point of synchronisation is the date of award of the doctoral degree, and this is what has been chosen in the rules.

Some additional advantages:


If applications were allowed before the doctoral degree was awarded, there would inevitably be cases where someone didn't complete the degree before their fellowship was supposed to start. 
All applicants should have a concrete body of work that can be used to assess their suitability for a fellowship.


Finally, while I don't know the statistics for Marie Curie specifically, in general success rates are skewed towards more experienced applicants. It may well be that the people impacted by this rule are generally not competitive for the fellowships.