I am betting somewhere along the way Eldritch Blast was changed to having multiple attacks while the invocation's wording remained unchanged.They are strong because of it but an invocation is a huge cost. There are lots of other great things a Warlock can get besides making a cantrip better.
In practical use the chance to apply it to multiple hits isn't as strong as it appears.
Level 1-4 it doesn't apply.
Level 5-10 you only get 2 shots and both aren't going to hit most of the time.
Levels 11+ the tier jump makes cantrips even worse than before. The Warlock gets a lot more spell slots. Most of their effectiveness is going to come from casting many 5th level spells to take on the big threats of the higher tiers.
Cantrips get worse as levels increase. They're at their strongest at levels 1 and 5. Every level above 5 they get worse. For example, at level 10 they're just as good as at level 5 but the threats have increased and the other resources the characters have, have also increased. At level 11 when cantrips get a boost spellcasters start having enough spells where they won't be using cantrips often.
An invocation to buff a cantrip at these levels better do something good.
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