You can't fit any number in such a little place.
And precision is not relevant facing such a big number
Finally, I think it hurts performance, so it may be a good design choice to annoy enough on order to incite people to close them :)
I thought that but then I recently ran Kiwi and Ice Weasel to over 100 tabs and got a 3 digit count. (note: it might have been just one of those but pretty sure it was both).