I can think of 3 reasons:
1) Price. nVidia is charging more than ever for entry level gaming cards, yet they're using VRAM size as a way for product segmentation. There have been articles in the past couple of years on TH about modders having upped the VRAM on these cards and shown a not-insignificant gain, so the performance of these cards is artificially being gimped by nVidia.
2) More and more games are using near or above 8GB VRAM at 1920x1080, especially when ray tracing is enabled. Techspot did a decent article about it last year, I suggest you give it a read.
3) DLSS frame generation. Those extra frames have to have memory to fit in, and I'll borrow a chart from that Techspot article to demonstrate, those extra frames can easily push VRAM usage over 8GB at 1920x1080, so imagine what memory usage will be once DLSS4's additional frames add to that.
4)