Yeah, it is completely dumb, but it is what it is..
The problem is that whastapp is old and it was first designed those decisions made sense..
the only other option around was SMS but it was expensive in many countries.. So now you had the option to pay a few dollars to buy the whatsapp (yeah, had to pay for it back then) and you could send SMS-like messages for free..
That is why it is so widespread in many countries, because it was a cheap alternative to SMS..
It was designed as a replacement for SMS and many of the early technical decisions that are limiting then today were made with that in mind..
One phone number = one phone = one whatsapp account
that is no longer the case today but they have to deal with the technical debts of those early decisions..
AFAIK all the other alternatives available today came much later with a completely different mind set, so they do not have those same limitations..