Indian Pacific Wheel Race - IndyPac - IPWR

Ultan Coyle used one in TCR 2 or 3 (I can't remember which). Position was just too extreme for so much time on the bike. Super stiff and narrow tyres probably not great either.

It’s 5500km. Relative comfort kicks in for me after 300km so... fuck... that...

emergency bivvy

IS sleeping gear.

Why don’t people use time trial bikes for these races?

Definitely doable but a few problems I considered:
possibly limited position (my Shiv would work but some bikes not tall enough to allow sustainable position)
limited tyre clearance - fat tyres really help on ultra races
they're often smaller or odd shaped so fitting 'luggage' becomes a problem
they're covered in proprietary bits and bobs so if something fails it's gonna be very difficult to fix (tricky cable routing etc)
gains aren't actually that big, especially once you cover the aero frame in luggage

Why don’t people use time trial bikes for these races? It seems those Titanium bikes with relaxed geometries are just a trend?

Agree - there is a lot of herd thinking, some sponsored riders and lots of people get the same kit to show group affiliation. TBH I've never really seen the point of Ti frames for anything much. But if people like them it is up to them, of course.

A TT bike would make sense for IndyPac if you could get the position ok for extended use. Christoph Strasser used one when he went across the nullarbor last year.

I didn't use mine as I've not been comfortable on it for a while and tyre clearance and carrying enough luggage + water were issues. I did use the wheels off my TT bike, though

For TCR there is more rough surface and more climbing and descending so less benefit. DK about TABR, I'd have thought all the windy plains would favour it.