Usually using poor quality cells. Replace with genuine panasonic 18650's with overload protection and you're fine.
Even just looking at the parts cost, are you sure you would still save money compared to going with the original battery? And of course you have to add your own time to this (if you actually like doing this, then it'll be cheap I guess) and the value of the warranty that you certainly will have voided at that point.
I totally understand that this might be a fun project to do and it will certainly be a nice achievement when you're through, but can this really be rationalised with cost of the original part?
Apple systems are generally better off in this regard as vendors like iFixit can make official batteries available for older systems, but for other vendors, no such luck. For example: I can't find any source for an official Lenovo battery for my first-gen X1 Carbon. It's also long since out of warranty.
There is still the time consideration, but assuming you have time available to do the work, why not?