ISPs are "natural monopolies" like water or electrical companies, especially in areas with low density. This means that even a level playing field was not enough, because small competitors can't afford the cost of building their own infrastructure (nor would we want them to lay redundant cables). Measures like Net Neutrality are ways of keeping the natural monopolies from abusing their position to gain advantage in other markets.

Here Biden clearly seems to be for net neutrality while Trump is, obviously, against it. So it's not as if this issue is not visible to both candidates. It's just never spoken about in terms of "monopoly" because there's no clear way to eliminate natural monopoly without a ton of regulation (e.g. sharing the last mile) or outright public takeover of the last mile infrastructure.