@Harvester methinks his reasoning is running along the lines of "If I get hit I die". He's only Lv 1, so he must have thought that even if he were to put all his points into HP/DEF/VIT or WIS/MP, he'd end up having a subpar build because he can only take a few hits before fainting. Therefore, his logic asserts that it's better not to get hit at all. This kind of thinking reveals that he focuses on self-sustainability in order to help comrades ("Can't heal if you're dead") which is why I kind of agree with the EVA/HEAL build, because it helps his teammates and himself manage enemies better, especially hordes of them.
This is because in a party, the offensive core won't need to protect him because his EVA stat practically guarantees he won't get hit, ever. This means they can focus on killing the enemies, with the added benefit of not needing to worry about getting hurt because, surprise, the healer's casting Curaga from long range while casually doing high-level gymnastics against an orc. What I think you're focusing on is the assumption that he WILL get hit at one point. The problem, I believe, with this kind of thinking is that it sort of fails to take into account his teammates thinning out the enemies so that the likelihood of him getting hit gets lowered from an already improbable chance to practically nil.
On him wondering why everybody thinks he's weak, refer to Clad95150's comment. Really, he should have known that was gonna happen when you say you're a healer with trash heals.
Also, the king is trash. Can't wait to wait 5 years to see him get upended then killed before the three heroes get reverse isekai'd back to their world.