I live in Japan as well and this it the thing I hate the most! You have half a shower and half a bathtub (unless you go very high end). I'd rather prefer a full shower or a full bathtub instead. Same happens with the toilet, it's a claustrophobic "room" under 1 sqmt, smaller than a ryanair seat. I would strongly prefer either having a bigger shower (no bathtub) + bigger toilet, or a bigger bathtub (no shower floor) + bigger toilet!
Though some Japanese people really want/need a bathtub, so I guess it's just cultural.
Shower + bathtub style is unacceptable for traditional Japanese lifestyle. All people on family take a bath everyday after a shower. They shares same bath water on a day because replacing water on a bath every time is huge waste of water and energy.
It's fine for single person who don't take a bath but just a shower, but it seems to not majority.
Most Japanese hates shower/bathtub + toilet style because it seems to completely nonsense, small space is relatively not a problem. While bathing, why should we see toilet and sniff toilet smell? Also toilet paper and cloths wets by bath. It's more problem for a family because Japanese house very rarely has multiple shower rooms.
Yes, I understand that the dynamics might be different for a family. In Spain normally you'd have 1 bathroom for each 1-2 rooms, so a house/appt with 3 rooms would normally have 2 bathrooms.
I am talking mainly as a single person renting. And of course it's my opinion coming from a different culture.
"While bathing, why should we see toilet" I prefer to see an ample room while doing anything, than to have a wall right in front of me. It's not about the contents of the room, but about having walls less than 1 feet on each side of my face (for the toilet). "Toilet smell" should not be a problem normally with decent ventilation, specially with the industrial fans that Japanese homes seem to have in the bathrooms.
My issue is with overall distribution, where Japanese houses (at least for single person/room) seem to take 30~40% of the floorplan space in what in other countries would be a single room taking 15~20%.
As the others said - you get clean in the shower (or, traditionally, by sitting on a stool and washing yourself with water taken from the tub - not a tub really, a round or square relatively deep bath). When you're as clean as can be you sink your body into a very hot bath. With modern baths you can set the temperature to be exactly what you want. I'm fine with 41C, my wife (as can most Japanese) can manage hotter water and prefers at least 42C except in the summer - cooler is better then.
The long hot bath every evening is vital.. Japanese houses are cold in the winter. No insulation, no bedroom heating, barely above freezing. If I don't take a bath at night I'll be so cold that I'll have trouble sleeping. But after a long bath I'm fine. Not feeling the cold at all and this lasts all night. Seems to help with my health too. No pains, colds are rare. Very much unlike like now, when I'm in my home country waiting for travels to open again.
The tub is for relaxing. The wash area is for washing. The Japanese tradition is first wash outside the tub. When you clean, climb in the tub and relax (like a Jacuzzi)
You don't have to go "very" high end. Nearly everyone I know who isn't under 35 lives in a 2 bedroom apartment with a tub larger than most western tubs and a shower (wash area) which just as much space as that tub
That's tub is the same length and wider than any of the 25 places I've lived in the USA and the shower would just be an attachment in the tub and I wouldn't have the wash area next to the tub.
Though some Japanese people really want/need a bathtub, so I guess it's just cultural.