[wmil]

Hearthstone is done in Unity. Unity has a bunch of 2d platformer features these days.

What you're missing is that these big engines are also game development environments with extensive tools and integrated market places to purchase assets and features.

Learning Unity is learning an entire platform. It's a marketable skill.

Generally smaller developers don't have the development resources to roll custom everything. But modern platforms have enough spare hardware resources that the inefficiencies of Unity are worthwhile.