That's the way it is. Bolt action rifles are more effective than machineguns and thus their suppressive effect should be more substantial.
i don't think you even understand the concept of suppression, we are not talking about the accuracy of the weapons. we are talking about what happens when the bullets pass close to you.
if i follow your logic than 1 rifle shot in your direction is supposed to be more suppressive than 200 rifles shots fired in your direction in 5 seconds time

like an MG would do.
a bullet is a bullet, a bullet will not suppress more when it comes from a sniper rifle, than when it comes from an MG. though the effect may possibly variate depending on the bullet type.
what creates suppression has nothing to do with the type of weapon that is used, what matters is:
-bullet type, larger bullets may make a stronger shock wave/sound when they pass you
-distance at wich the bullet passes you, the closer to you, the more you feel it
-amount of bullets, high fire-power means you have more suppressive effect
that is what matters, so yes an MG 42 burst will have a much larger suppressive effect than 1 rifle shot.
something you may understand better lol, is that in movies they oftenly scream "pin them down!" or "suppressive fire!", in your logic the sniper and bolters then start shooting while the MG's storm the building?

the use of the MG is that 1 person with a large amount of fire power can use that fire-power to "shoot for the others" while they are advancing and not able to fire. his goal is never to kill the enemy, but to pin them down so they won't shoot at his squad members as they expose themselves to advance.