Produce grown in the Land of Israel has to have Teruma, two of three different kinds of Ma'aser, and Terumat Ma'aser removed, as described in this OU Kosher article. Of these five separations, three - Teruma, Ma'aser Sheini, and Terumat Ma'aser - can only be eaten in a state of purity which is unavailable today, so it makes sense to do what we can to minimize or redeem them.
The other two - Ma'aser Rishon and Ma'aser 'Ani - are meant to be given to Levites and poor people, respectively. However, according to Footnote 7 of the article linked above, the original owner of the food may keep it, because these separations have no inherent sanctity, these gifts only constitute monetary obligations to the potential recipients, and there's enough of a doubt about the obligation that they couldn't successfully demand their due in court ("hamotzi meichaveiro - 'alav hara'aya").
It seems to me that what this doubt establishes is that it's possible to evade having to give these gifts to their intended recipients. However, why is this (as I understand it is) standard practice? Isn't it avoiding a chance to do Mitzvot and also undermining the clear intent of these Mitzvot - to provide for Levites and poor people?
I understand that if I buy a Jaffa orange in the US, it's impractical and undesirable for me to try to bestow one of the segments on a local Levite or poor person. But wouldn't it make sense for food producers or even home gardeners in Israel who want to live in accordance with the Torah as much as possible to follow through with these Mitzvot as originally commanded and practiced? (Or do some actually do this?)