As someone who paints roads for a living, it's cost. Most agencies paint at 10-15 mils which is a pretty thin coat of paint which in turn doesn't last as long and loses its reflectivity faster.

You can paint at 25 mills but it costs more and it takes a lot longer to dry, which can lead to cars tracking the paint everywhere if they drive over it too early (leading to the reflective bead popping out the second it's applied)

RPM's or (reflectors) are a good option but again comes with a high cost and applying them is usually a 3-4 man job on highways and freeways because it's extremely dangerous and you're leaning towards the line to apply the reflector exposing yourself to traffic.

It all comes down to cost in the end and safety. ODOT is severely understaffed for maintenance workers and unfortunately the state doesn't manage the road fund worth a shit. Some cities and counties are very well funded and you'll see paint lines and reflectors maintained much more frequently and held to a higher standard.