
Because when everyone knows that your only play is to support the reds, then the reds themselves know that they can abuse that desperation, renege on deals with you, etc. After all, what other plays do you have?
Dropping the deal is short-term disadvantageous, but by establishing a reputation for punishing allies who don’t uphold their end of a bargain, they can be more influential in the future.
You might disagree with the argument, but it’s not completely random. From the US, we gained most of the benefits of having nukes and spooks, without having to maintain them ourselves. Since we can’t trust them, we now have to decide if it’s worth developing our own.