Higher warp speed are taxing to the ship's reactors and fuel injectors. In other words they would run out of gas very quickly
Think of it this way:
How often do race cars --F1, Open Wheel & NASCAR vehicles-- have to get their tires changed over a race because they're almost always driving at near 321 + km/h? They change them a lot because they get used up, because they drive at full throttle the majority of the time.
Starting in The Next Generation Era, the visual effect of going to warp, makes a ship look like a rubber band being stretched & released. So if you stretch it further back, to engage the warp engines at warp 9 + all the time, it will break sooner than if you start at warp 1 & gradually go up.TrekFan1984 Ends (talk) 21:44, May 6, 2017 (UTC)
An example of this can be seen in Best of Both Worlds. The Enterprise pursued the Borg Cube at maximum warp for so long that it had to stop to repair the engines and ended up being late for the disastrous Battle of Wolf 359. The ship can only manage a few hours at maximum warp, but can maintain lower speeds for significantly longer.-Cpthunt (talk) 05:18, May 11, 2017 (UTC)