They're not going to build a 4 mile siding, which is the length that many freight operate at. At that point it's like building a second set of tracks.
Then don't build 4 mile trains. It needs to be possible for trains to pass each other.
a lot of rail is 2 track so no sidings are needed.
Because trains never go the other direction anyway.
Dual track isn’t really a two way road, with signaling it ends up being infinite sidings, and is used almost more for passing than for two-way traffic - if it exists outside cities.
Unfortunately much of the USA is single or effectively single tracked.