No, it's not at all like that. Knife users will be perfectly capable of understanding what they're doing when they cut off their fingers, and assessing its impact on them. They don't even need pages of warning text to explain this.

That's not the case for smartphone (etc) users.

A lot of tools have safety measures which can't be circumvented by their users (e.g., you have to use both hands to start thems). The reason being that some dangers are easily underestimated, even by experienced users. Manufacturers do indeed much better about the inherent risks of their products than users.

If a knive could be built which allows to cut food, and protects you from cutting off your index finger, wouldn't that be great?

Are you really comparing an object which holds a threat of blood loss, loss of organs and possible death as requiring similar safeguards as a phone?

"Appropriately warns people" is nearly impossible and shouldn't be brushed away as a non issue.