High-end TKL builders: why?

[AMongoose]

Personally, I feel like F-keys are a relic from the DOS era before everyone had a mouse and drop-down menus.

I mostly agree. I think that Function key rows are suitable for hunt-and-peck typists only. Reaching them requires a larger motion so it is difficult to acquire muscle-memory for them. The goal should be to help typists type without looking at the keyboard and therefore they only repress the development of keyboarding skills.

LOL

I touch type and I can reach F keys without looking and without a single mistake. I use them continuously for autocad and for graphic software. I never have to look at the keyboard. The fact that they're spaced makes any error impossible.

This.

Also any visual studio developer relies heavily on F keys, Android studio to.

And having a mouse in the right hand makes sense to have the cursor keys on the right because you don't use both at the same time, the numpad could be on the left tho.

[Puddsy]

i use F keys in scrivener to navigate quickly, and then some for one-stroke bolding and underlining

[Zobeid Zuma]

i use F keys in scrivener to navigate quickly, and then some for one-stroke bolding and underlining

I have been using Scrivener for several years, and I never even was aware that it has any F-key shortcuts assigned.  In fact, I'm looking through the drop-down menus right now, and I'm not seeing any F-key shortcuts.  Bold is ⌘B and underlining is ⌘U.  Are we even talking about the same program, I wonder?

[emenelopee]

Likewise, I think if you have a mouse on the right, then it would make more sense to have your nav keys on the left.

I'm not sure that I agree that arrow and nav keys would not belong on the right side. I use those keys mostly when I type, not together with the mouse. Otherwise I scroll using either the (right-handed) mouse or (right-handed) keys. Even if they would move, I think that the (forward) Delete key belongs close to Backspace.
I think that a staggered non-split QWERTY alphabetic area does not actually belong in the middle in front of the user but slightly to the left because of how the key staggering leans left.
When I sit straight in front of the desk with my arms stretched straight forward, my left hand is on the left keyboard half and my right hand is on the mouse. The arrow/nav cluster is not in the way at all.

And on the flipside, I disagree with you  . My custom is all about putting the nav/number clusters on the left for symmetry and mouse space, and getting rid of the key stagger centres everything. My use case is with nav/mouse interchangeably. I also put the backspace onto the nav cluster for the reason you state. In fact the reason I made my own is because TKLs were not doing it for me with no number cluster and with nav redundantly/uncomfortably close to the mouse.