75-Ohm Coax cable can absolutely do 100 up. It can do gigabit up if you like.
The issue is that, on most systems, there are 'diplexers'... amplifiers that amplify in one direction over a certain frequency and the other way below it. And the million-or-so of them in the current networks are mostly set to amplify out everything starting at 54 MHz up to 1 GHz or so. Because 54 MHz was the start of 'Channel 2', as defined in 1948 for broadcasting Amos n Andy in black and white, that's the standard still today.
For technical reasons, this left only a narrow window from ~15 MHz to 42 MHz as workable upstream bandwidth. Which caps out around 35-40 MBps and why you see few cable operators going beyond that.
They've known for about 20 years now that they need to pack downstream a bit tighter, move that 'split' to 150 MHz or 250 MHz, and they can accommodate Gig-plus down and near-Gig up speeds. But, it's expensive (all those pole-mounted amps). They're too busy counting their profits. So, 20 years on, few have done it.