We live in America. We have an old house (>100 yrs). One bathroom had a tub. We renovated the bathroom to incorporate a shower head. Turns out we couldn't purchase a new tub that fit the old dimensions, as new tubs are smaller, reportedly to meet new efficiency standards. So we had to frame in a smaller recess to fit the new, smaller tub. Wah wah, I guess the answer is, old tubs used to be more adult sized, and new ones aren't because of efficiency / climate change trends? As others have pointed out, you can still get big tubs but they're more like jacuzzi or whirlpool types that take up more space.

Our new home came with a pre-installed freestanding bathtub in the master bathroom. Fits an adult easily. But I don’t think you can fit one through the door - it would have to be taken apart for sure. It’ll be a big reno project.

My house built in 2017 is the same. I still don't really use the tub. I asked my wife why she never uses it, and she said "it takes forever to fill."

Japan has the solution to this. Tubs are connected to a smart system and you can start your bath from, say, the living room via a panel. You can specify what time you want it filled up at and at what temp and it does the rest.

Some tubs even support re-heating the existing water, so you can re-use the same bathwater the next day or later in the evening for other family members. (in Japan it's assumed you're going to shower and clean your body throughly before entering the bath, like in a hot spring/public pool, so the water doesn't get so gross)

Measure the height. They often just barely fit if coming in sideways.

But the large “one piece” bathtub/shower combos you see at the hardware stores definitely are installed early and won’t fit in the door. They are more leakproof but they sell remodel versions that come in multiple parts.

FWIW, my spouse works in rehab hospital settings and says tub accessibility is a big problem with lots of the much older and more modern freestanding tubs. Relatively thin, high sides are an injury risk for a lot of people. It's one reason many tubs are shorter and have wide flat sides.

The struggle is real. I hope to one day own a bath tub I can fit into comfortably. Half of me is always popping out.

FYI hot-tubs are basically bathtubs but better.

Of course. It's obvious why....

The bath tubs are too small.

I got laughed at by a north american once for letting her know I was taking a bath (instead of a shower). apparently "bubble baths" are only for kids there. In austria either are common though. Modern bathtubs are usually a bit slimmer to save on water.