Probably because that wouldn't be straightforward. You'll have every situation from missing licences, licences for old companies that don't exist anymore but wh

Probably because that wouldn't be straightforward. You'll have every situation from missing licences, licences for old companies that don't exist anymore but whose IP has been absorbed by a patent troll, companies who refuse to be negotiated with, companies who will take the opportunity to file suit to repay the incalculable harm to their bottom line (despite not knowing their software was in Windows for 20+ years), companies who would be up for negotiation but don't have the momentum or enough of a grip to see negotiations through without collapsing or trailing off. And this is if Microsoft DID want to open source it, which if they were giving in to a demand from FSF, or trying to do it as part of a goodwill "Microsoft <3 Linux" initiative then you will quickly find that the costs far outweight the benefits.