I think people are incorrectly thinking that only freestanding ovens come in 900mm size.
You can get a 900mm built in oven.
Me, having built two new kitchens, one with a 900mm freestanding cooker and one with built in cook top + 600mm oven, I much prefer the latter. I prefer it without the gaps. But next time will get 2 x 600mm ovens.
Also 900mm are slower to heat up.
It's different if you have an old country style kitchen and get a Falcon freestanding cooker.