Why are specific 'game engines' requested by publishers when looking for html5 games?

[Ritual]

I have noticed a trend on this site where various publishers want html5 content but only if it is developed in specific engines like PixiJs or Phaser etc, why is this? I am not seeing why it should matter to the publisher what you used to create the game, so long as it is actually html5 and runs exactly as intended on the targeted platforms (be it mobile / desktop or both). Do publishers of html5 games intend to make their own edits / changes to games they license so require that they are developed using the same software they use?

[icp]

Yes, there are publishers that implement their own API, make reskins and add features if a particular customer has a request for them. So it can be a mess to implement your API/Ads in a codebase that you don't understand (example: Video Ad between levels in a non-native HTML5 exported game with tools like Construct, Unity, LibGDX. This would require publishers to build their own plugins).

[Ritual]

I am surprised that html5 devs sell licenses that allow the publisher to reskin their game and make other such changes themselves! Back in the days when Flash was used the dev's would implement the publishers API and provide them with the finished product, it didn't matter what you used to develop the game so long as you provided the .swf file with the required API running properly.