system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40K] What is the purpose of a space marine dreadnaught? Why not purposely build mechs for their normal marines as well?
gpt
1. they're expensive to build and maintain. 

2. Space marines can survive MASSIVE damage, due to the process that transforms them from mortal to massive demi-human warrior. More damage than a normal human would survive. They go into protective comas, bodies shut down for a while instead of just dying, allowing Apothecaries to recover the body and salvage what would have been dogfood if they'd been human. that extra time for body recovery means that Space Marines survive long enough without medical support to get them back to a facility where triage can be safely performed. 

3. The problem with being able to survive massive damage is that, over time, there's less and less marine to rebuild. After a certain point, it's easier to build a Dreadnought around the marine's remains than it is to build a mechanicum level cyborg out of them. (even more expensive, requiring years of painful, slow augmentation and the surrender of more and more flesh).

4. Not even every space marine is able to survive the process. 

5. Not even every space marine is WORTHY of the honor of being held between life and death in a combat-rated cryogenic suspension chamber, basically a goo-filled armored fishbowl. The only SM worthy of the cost, effort, and "honor" are those the Chapter/Legion cannot do without - Those whose knowledge, experience, and/or sheer lethality are both legendary, and irreplaceable.  For Example, IF Brother Schmendrick, Sub-Doosh of the crimson Mustachio chapter, known for his last-in, first out combat strategy, champion complainer, master of the humido stragulum technique, and earner of no honors in his time serving the Emperor of Man, manages to step on a land mine, his chaptermates will not hesitate to harvest his gene seed. He might still be alive, having survived losing everything below the ribcage, both arms, and his left nostril, and about 20% of his colon, but his brothers will not waste time with him when Seargeant Gorram GulfWighter, Master at arms and Hero of the Imperium, has just lost his fifth set of augmetic legs to the same blast, and must be saved so that his knowledge can live on.