ROOOOOOOOOAR!
Okay here is the dig with this issue from what I understand. It’s really a matter of doing business with the console manufacturers (more than it is a technical issue). Any item that is acquired through any form of in-game currency (Gold Coins 
, OWL Tokens 
, Competitive Points 
), purchased with real money (the Pink Mercy skin is a good example) or through loot boxes are restricted to each platform that you acquire them on. The main reason for this is that most of these items can be acquired optionally through a cash purchase in the appropriate store of that platform (BattleNet Shop for PC, PlayStation Store for PlayStation 4, and Microsoft Store for Xbox One).
Items that are acquired through a promotion can be sent to each platform that is linked to the BattleNet account the promotion is redeemed under. For example when you earn Overwatch League Tokens , you earn it through all three platforms, but the items that you can spend Overwatch League tokens on are not shareable because you can purchase Overwatch League Tokens manually with cash. Same goes for acquiring items from Loot Boxes because you can purchase Loot Boxes.
If this was just a matter of Blizzard setting the rules, we’d probably be able to merge or transfer accounts across platforms. The problem from what I have seen across multiple games (not just Overwatch) is that consoles do not want to you leave their platform, and because of this they make the licensing agreements with each third-party developer to restrict sharing account content across platforms. Its kind of the same reason why cross-play is not yet a thing for most games including Overwatch.
The good news is that these barriers are starting to get torn down (with games like Fortnite being a notable example). We do know that the Overwatch Dev Team wishes they could allow cross-platform account transfers/merging. There may be technical hurdles but from what I know this is more of the matter of licensing among the console manufacturers.
Source: Overwatch Forums
Personally, I am hopeful that one day everyone can transfer their accounts to the platform of choice. I rather see this as I think this would keep individual marketplaces and platforms competitive and be innovative, rather than just try to host a monopoly.