Embrace, extend, extinguish: https://en.wikipedia.org/wiki/Embrace%2C_extend_and_extingui...
But, there's a lot of value in having an open source and libre edition of anything which started out or is maintained as a closed-source, proprietary system.
Consider: Firefox versus IE, Linux versus OSX or Windows, or LibreOffice versus Microsoft Office.
With Minecraft, there is no opportunity for developers to really get on-board to build stuff with them. The source is closed and Mojang is AWOL when it comes to the developer API.
Minetest, on the other hand, is LGPL and is freely available.