It's more on the electronics and software side than drilling and grinding metal side, which I think are easier and cheaper equipment wise. Also, lots of parts you can print and then bolt on or slide into t-slots.

The Ender 3 is kinda like a parts kit of an open source design but all in one package and provided with low margin or benefiting from economies of scale (being cheaper than you can buy/build yourself).

I looked into just building a RepRap but it seems they were more pricy.