They ... do let you do that. Being able to make changes to the code and redistribute your changes is literally the whole point of open source, and the popular licenses don't contain restrictions on making money (nor could they, if they want to fit the Open Source Initiative's definition of open source).

The permissive open-source licenses (MIT, BSD, ...) let you just take the code, make whatever changes you want, package it up into some proprietary product, and sell that product. The only requirements are usually that you maintain some attribution notice. The MIT license, for example, says this: "Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software".

The copyleft licenses (GPL, LGPL, ...) lets you take the code, make whatever changes you want, package it up into some product, and sell that product; but you need to provide your product's source code to your customers.

Show me an open source license which prohibits me from taking the source code and selling a product based on it.

The commonly used licenses like GPL and MIT don't put any restrictions on commercial use granted you don't break any of the conditions and limitations.

Theoretically you could even just take any software using any of those licenses, optionally package it up and then commercially redistribute it as is and it would not breach the license. Needless to say people probably wouldn't look at you favorably in that case though.

You typically only do that the the intended API used for modders, would guess that is what is done here?

Not quite the same thing.

You obviously haven't battled the mod game by rebuilding and fixing your mods from zero every time a new version comes out.

The good tooling is from the community. Not majong and especially not Microsoft.