No sense building a hot rod or custom car for all the money you will spend, hours working on it, the tools, etc. Just go buy one.
Then there are those of us who understand exactly why we chose the parts we did, the amount of work to get them to interact dynamically, and how to exploit the results. This isn't about Glock per se, AR's and now the SIG P320 are available as 80% lowers.
Take the AR for example - if I chose a flat kit to assemble, I would have the option of also shortening the mag well, adding a flat strip trigger guard, moving the grip mount position to the rear 3/4", and having the "finished" result be more ergonomic, and not look like an AR lower. Something more like an AR180.
With AR's and Glocks the serial numbered part being the FFL one, sidestepping that exercise in public regulation is very attractive to some, who can then also point with pride of workmanship in the finished part and having an operating gun. The lack of serial number is of no real importance to most. Actually building a gun that works is.
With the Glock or AR, the result is finished gun, and yes, it will cost more because buying the parts at retail one at a time nets no discounts the major brands enjoy buying parts 1000 at a time. It is a given. However, if you buy a complete OEM stock gun, then swap out the parts you don't want, you get the same result in terms of money. Even unfired, the spare parts are "used" as they have marks on them from being assembled and taken apart, plus they are the boring cheap commodity pieces everybody else sells. Building a gun is about putting together your vision of what you want that nobody else offers.
I've seen few vendors offer an AR pistol upper that takes carbine handguards. It's all about free floats - which is patently ridiculous as there is little room for anything, and the effective range is so close that "precision" shooting isn't the point at all. I built my own and it's not only unique, the polymer handguards are warm to the touch in winter, and regulations don't allow lights during hunting season anyway. That is just one example of why people assemble their own.
There is also the impending regulatory action - which the anti gunners did pass in a western state - where the transfer of any firearm requires a background check. Even loaning a gun to your son for hunting might be included. With an 80% lower, no serial number, that becomes a laughing stock. By making them now, it completely undercuts the intent of that law which is meant to restrict firearms transfers between citizens and intrude on their property rights with government oversight. Frankly, we need a few million MORE unregistered firearms with no serial numbers. It will stop the gun grabbers by the sheer futility of making new schemes to control them and force them into a corner where they have finally fall on their end game - total confiscation.
A working lower, or grip unit for a Glock, might be 50% more expensive as an 80% lower, but in the overall cost of assembling the entire gun, it's a modest increase. For AR's, a free float can run upwards of $400, and there is also a premium when choosing an alternate cartridge. In the Glock parts bin, a Zev custom slide is more than the cost of a complete Glock - but you can't get one from Glock, ever. If you want that as a carry gun, you have no choice - the cost of the 80% lower is basically incremental at that point. If you have skills and can do it - why not?
So, it's not about a 80% lower making the gun twice as expensive after all. And it IS about being able to freely exercise your constitutional rights to keep and bear arms. The fundamental fact is that in the day, guns in America were manufactured here because we could exercise the freedom and were making better, more accurate ones, too.
As for me, nobody makes a short mag well AR lower in aluminum flats, binary trigger, and extended reach grip. I can - and then use it for a pistol lower with a bolt that has an integrated buffer which removes from the end. These are the kinds of projects you can get into by building your own 80%. It's no different than putting a Hemi in a Studebaker - can't buy that off the car lot - and we don't want or need government oversight to do it. Unless, like Australia, you prefer for an Engineer to examine your mods and then sign off with a hefty fee certifying your headers and 1" larger wheels are safe on the public roads?
Look at how heavily cars are regulated here and abroad and ask if you want that for guns.