Hazard a guess, it's pretty simple. Either:
A) their marketing email system not only doesn't have dynamic access to their frequent flier balances, it doesn't even have access or code to know what people already purchased from the miles partners from the same offer,
or
B) they don't have the time, money, or competence to get someone to think about the logic that should say "don't offer to someone if miles_purchased_2019 > max_dollars"
or
C) the case described by the OP is such a corner case (so few people buy miles, and fewer enough miles to be impacted by the limit) that they simply decided it was not worth the effort to code the logic to handle the case.
(just yesterday in the UA forum I came across a thread where a poster was told by many regulars that the conventional wisdom is that large miles purchases are a bad deal and only to be used to top off the balance. I don’t necessarily agree with that, but...)