Listening to APP for several major (US class B) airports, it seems that every flight is vectored to intercept the localizer. Is it really possible that 100% of flights filed an ILS approach? If so, why? It seems like at least some (perhaps small) fraction would have filed an RNAV approach. Can someone with only GPS still "intercept the localizer" by flying the given vector until the runway is in sight and then get cleared for visual approach, so actual ILS doesn't matter? Or does that case just never come up because ATC knows (via the equipment code in the flight plan) who has ILS equipment available and makes them use it even if that's not what they filed?
As egid pointed out, pilots typically do not file a specific approach as part of their flight plan. ATC will decide what runways and approaches are in use based on the weather. As the pilots are on their descent to the destination, they will be told which approach to expect. Pilots can always request something different but ATC is trying to get everyone to their destinations as safely and quickly as possible.
It's easier to space and sequence aircraft if they are all on the same approach, rather than trying to get aircraft on multiple approaches to mesh together just right. It's also easier to sequence them if they are all on the same approach course along the localizer, where an RNAV approach may have multiple initial fixes that follow various paths to get to the same final approach fix.
An ILS approach gives ATC more control over the spacing and sequencing of aircraft onto the final approach course. Aircraft can be vectored in anywhere along the localizer. RNAV approaches have more locations where aircraft can join them, which may be inconvenient depending on the direction from which an aircraft is arriving. Not all aircraft may be able to join an RNAV course by vectors, but pretty much any airliner can intercept an ILS localizer.
The same benefits can apply even in visual conditions. The localizer provides a common reference that all pilots can join and follow while they can maintain visual separation from each other, allowing closer spacing of aircraft. It's easier to say "join the localizer" than "join the RNAV RWY 17 final approach course."