Typically the limiting factor on your phone is the screen breaking, your battery life getting too short, wear and tear on components like buttons or the charging port, and factory defects. Epoxy isn't going to help with any of those. The only thing it would help with is exposure to water, but if other parts of your phone like your screen aren't water proof, what's the point?
Epoxy adds weight and manufacturing cost. It introduces design challenges as you need to balance the thermal expansion of the various parts. It's an extra step that can go wrong, and makes repair of other defects far more difficult. What benefit is there for the typical consumer that outweighs these costs?