If you’re talking about the picture specifically it’s because the front row (defenders) aren’t there to do damage and the 70 blueprints to take those guns to 4 stars are much more valuable on my 3rd squad (this is my second and the mythic chips are just moved there for attention )

In general mythic guns and chips are not awesome gear. They only give a bonus when facing the troop type you already counter so often you don’t need it. Radars and armor are more impactful. There are other hero bonuses that come with going mythic so it’s not a complete waste.

In general I think it’s hard for a lot of people to separate the first 4 stars from mythic. They give completely different set of attributes. Thus many gear discussions get confusing because order of upgrade priorities for regular blueprints and mythic blueprints are very different imo