It isn't just about kits specifically. The principle is that engineer has stronger utility skills in general to offset the lack of weaponswap - if you're not taking kits, you're taking utility skills that are better for what you're planning to do than a kit would be. This is why, for example, mechanist signets do so much. Granted, there are some engineer utilities that have fallen behind, but at least in theory a kitless build isn't supposed to be so much better than a build with kits that weaponswap is needed to compensate.