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Because "left" works fine as the verb and doesn't require "is". To me, it flows better in speech with "is", but that's a subjective opinion and probably a culturally-influenced one; British people use that construction less than Americans like me, in my experience.

It would be more correct to add “was” since it happened in the past (the act of leaving it blank). Not necessary at all because “left blank” is already past tense.

This page intentionally left blank.

"is" intentionally omitted.

not to me.

does 'he eats faster than I [do]' seem wrong to you? it's a different problem ('than' as conjunction vs 'than' as preposition)[1] but to similar efect.

[1] https://www.quickanddirtytips.com/education/grammar/than-i-v...

To me, "This page has been left blank intentionally" would sound the best, but eh.

"page intentionally left blank" seems kosher to me

Omit needless words?