Hi lebaryo,
Primarily because it’s a high risk and possibly costly proposition and I have seen the dissappointed results of many consumers who believed that it was easier that it really was and ended up regretting their choice.
It has nothing to do with the membership here or where someone buys a mattress from (see post #5 here) but about the risk being much higher than many people may realize. The goal of this site is to give people accurate and meaningful information that helps to create reasonable expectations and trying to design and build your own mattress is just higher risk. For those that are aware of this and are willing to take more risks for the potential of more rewards (or lower cost) it can be a good direction to go.
The goal of the site more than anything is too help people find a mattress that has the best possible quality and value and that is suitable for their needs and preferences. At the end of the day when people look back on their mattress purchase most people will remember much more about how well they slept than they will about how much they paid. If a complete DIY is the “best” choice for your personal value equation then that is the 'best" direction for you to go … as long as your eyes are open.
If the type of latex and the latex manufacturer is the same, the ILD and thickness of all the layers are the same, and the cover and quilting is functionally the same, then you’re correct the two mattresses would be substantially the same. This would depend on having accurate information about both a mattress you are trying to copy and the materials you are using to copy it though. The difficulty is that it’s often not possible to “match” all the specs and components that can make a difference in the feel and performance of a mattress and many consumers don’t fully understand the effect that seemingly small differences can make (see post #2 here).
Phoenix