Why is the Marie Curie Individual Fellowship only for 'experienced researchers'? The Marie Curie Individual Fellowship (IF) has the following eligibility requirement: 

"Researchers must be experienced researchers (ER), i.e. at the date of the call deadline (for IF) ... be in possession of a doctoral degree or have at least four years of full-time equivalent research experience." (page 80 of the PDF Work Programme 2018-2020: Marie Skłodowska-Curie actions).

By my calculations, this means that that a student spending 4 or fewer years in graduate school must have a lag time of 6-12 months after obtaining the PhD before they could begin to recieve funding. Why is this desirable? Are there any other EU-wide fellowships for STEM which could fill the gap? 

(I am aware of other postdoc funding opportunities but none that are as prestigious.)

EDIT: To clarify, why are PhD students not eligible to apply in the fall of their final year, unless they already have "four years of full-time equivalent research experience" at that point? (i.e. a student graduating in May 2021 and applying in September 2020)
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.