I think Pact of the Chain makes sense as a cantrip - instead of a feature that allows the warlock to cast Find Familiar in a special way that significantly changes its functionality, just directly giving them a spell with the desired functionality makes sense, and frees the designers of the constraints of having to use Find Familiar as the baseline.
Pact of the Blade also works fine as a cantrip in my opinion. The main function when you first gain the pact boon is to allow the warlock to magically summon a special weapon at-will. Magically doing things at-will is exactly what cantrips are for, so sure, that works, and the structure of Cantrips already includes an “at higher levels” feature, making it easy to work the 5th level extra attack directly into the pact boon instead of needing an additional class feature.
Pact of the Tome makes a lot less sense as a cantrip. It’s a book that gives you access to three Cantrips and some number of ritual spells. Really, the “at higher levels” structural feature of Cantrips seems like the only reason to go this direction with it.