Reasons to DIY build a 3D printer

OK, only just at the end of last week, I was given an Ender 3 original, my very first 3D printer.

Having seen how crap the mechanical design is (why didn't they just make it out of soft rubber) I wondered about DIY builds, but after chatting with the guy who gave me the printer he said just buy one. I do a search online and seems many say you don't save much money over a ready made one.

However, I look at the price of larger printers and they seem to cost a few thousand.
My studies over just this weekend have shown me that the dedicated 3D printing mainboards are freely available, not that expensive, and quite configurable.
Firmware such as Klipper / Marlin is free. 
The slicing software is free.

So it seems it's just the price of the hardware/parts that is the main cost. And I know I would have a rigid and accurate build with good repeatability.
And if I can pick up an old used printer cheap I could use all the existing working parts on a quality built frame.

What's the experience of those who've built their own 3D printers. When it comes to a larger machine, say 450 cubic and up, do you find substantial cost savings or was it other reasons you went the DIY route.

I've had my Ender 3 Pro for a good time and have found it to be a good solid workhorse.
Far by the best mod was a glass build plate, was fighting with adhesion for a while, adhesion is that good now I have to use hairspray to be able to remove the printed item when it's all cooled down .
If I was to build my own I'd reduce the Z axis travel to about 150mm and probably go for about a 400x400 bed.
If I see "game changer" in a another YT review I'll be quite upset and may commit an act of unfathomable violence upon society.

Most printers are made of 80x20, which while it works in the short term, fails slowly over time. Bolts come loose, pom wheels (that ride in the 80x20) wear out, belts loose tension, etc.

If your gonna build your own buy 1k in parts and make it with ball screws, linear rails, nema 23 motors, 400mm+ etc, steel frame preferably welded.

It'll take a lot longer to setup but will last for decades

Cant say I've had any bolts come loose, Loctite will fix that if that's the case. The wheels are a consumable, but meh.

If you decide to build your own, look up marl rehorst's megamax design.

I'm about ready to mill a 315x315 core xy top plate from 10mm aluminum. Center cutout will be the bed. Similar to a e3d tool changer. I've had many of the parts for over a year.

I recently saw some big multi color prints done on a bamboo printer that were frankly amazing. If I didn't enjoy building my own printers, I'd probably just purchase the bamboo PS1 and save some money and an incredible amount of time.