Why is it a membrane keyboard, when previous models were not? Because it is much cheaper to make.
Why does the Model M has only loosely fitting case parts? Because it is much cheaper to make.
Why does the Model M has plastic rivets that hold the membrane stackup together which always break off due to ageing and bad design after 20 years or so? Because it is much cheaper to make.
Why does the Model M have a single-piece barrel plate with barrels that occasionally break off, rendering the entire keyboard garbage? Because it is much cheaper to make.
Why does the Model M have essentially zero spill resistance? Because the cheap design doesn't permit spill resistance. When you spill water on an M, expect to either have it dry for weeks or months as the water evaporates from between the membranes, or to disassemble it. If it wasn't pure mountain spring water, you have to disassemble it, which--due to plastic rivets-- is a destructive proecss. To reassemble it, you have to drill a bunch of new holes through the actual keyboard parts to put screws in.
Yet despite these significant design and longevity issues the M somehow got a legendary reputation for being "solid"... it really isn't.
Go look at Model F reproductions. Those are solid keyboards. https://www.modelfkeyboards.com/