Is 5G really that much more reliable than 4G? Because on 4G I often get spikes (of lengths between 2 seconds and several days) of low speeds, dropped packages, high latency, etc. Whereas I've never had an outage on fiber, nor has anyone else I know. Fiber is also not affected much by other peoples' usage.

I think fiber being shit is very much a regional thing, and mostly Northern American. It's the lack of competition. A good fiber service will outperform a good wireless service any day of the week. And it can be done on the cheap too, if there's competition. Romanians get gigabit fiber for like 8 euros a month and 300 mbps is the minimum speed offered I believe. The key is that you can't let a single ISP own the entire network in an area. Should be government-owned ideally.