MS and Apple have the same thing, they're just less successful. Just a browser and an OS was previously seen as antitrust (and it looks like MS is being anti-competitive in this space still). Just a browser and a search engine can allow anti-competitive behavior. Or just a search engine and an ads platform...
The problem is the anti-competitive behavior. Businesses are generally rational actors, so clearly our system isn't working. It's unclear what the boundaries are until years in court, and even then it only applies to a single company.