Let’s look at this from a different angle, you are assuming that almost everyone gives maaser. This might be true to some extent, but does everyone give maaser right away? Is everyone midakdek to make sure they are giving at least a tenth? Does everyone give it to the cause that it should be given to (aniyay ircha kodmin, etc)? Does everyone give it with a smile? Perhaps (I’m just guessing here) the bracha only applies to those who give maaser b’leiv shalem following the halacha exactly without cutting any corners, etc.
And getting back to the topic of this thread – segulos. Giving maaser seems to be a segula for “wealth”. Does this segula work? Does it require proper bitachon and davening as well in order to work? Maybe it only works if you really believe in it b’leiv shalem. Many people claim that they are big ba’alei bitachon when it comes to these types of things just as an excuse to have a tayna against Hashem. If you have true bitachon than this question probably won’t bother you.