Actually, they do.
And I bought mine a while ago and have yet to give birth so I can't really attest to it's usefulness there, but I sit on it and my 30 lb toddler holds onto my shoulders and jumps behind me and it hasn't popped yet.
All the birth ball websites I looked at said a 65cm circumference is the best size, so that is the size I bought.
Also, this is my last birth and I don't need something that is super durable, just something that is inexpensive and will get me through one labor. I planned to sit on a towel regardless since I most likely won't be wearing anything on my lower half, so that isn't an issue.
It came with a pump with a strange adapter that I don't see how you can put on a different pump. The pump is really small and it probably took me 1/2 hour to fill the ball so if you go this route, buy it early and pump it up now.
I've had mine for about 3-4 months and I have had to give it a little boost of air twice in that time, fyi.