This comparison again?
I quite often ride one of my bikes to and from work, golf, fishing, errands, etc. I quite often drive a tractor at work. I often switch between tractors during the day, depending on what I need to accomplish. There is no valid comparison. Tractors and bicycles, cars too, are governed by the same overall rules, but each has rules specific unto itself that does not apply to the other.
A tractor is much more stable moving over bumpy or loose surfaces, and much less likely to crash due to irregular surface features or debris.
Even a small tractor is much bigger than a bicycle and much more likely to be conspicuous to motorists, regardless of road position.
On a bike, I often do pull off the road and use a paved shoulder or bike lane to extend the courtesy of ease of passing to faster vehicles approaching from the rear,
if I have the available infrastructure to utilize in this manner,
and if I deem it is safe for me to do so.
Other cyclists that I see, which are rare in this area, never leave the paved shoulder or bike lane, not even when that is the much more hazardous road position for them to be in. When they don't have these facilities they ride in any lane, including oncoming, in which they don't currently have another vehicle approaching. When other vehicles are approaching from both directions they ride completely off the road. This riding style is usually undertaken by people on exclusive mountain bikes. Exclusive because they are available only at Wal-Mart. Just a few weeks ago a man on a bike got hit trying to execute this riding style.
When I'm driving, I expect a slower moving vehicle ahead of me to maintain their line, or at least stay in their lane. I can slow until it's safe to pass, and then execute a safe pass. It doesn't hurt.
Much better for a cyclist to be conspicuous and predictable than swerve all over, or even off, the road, trying to get out of the way.