Ma’aser Sheini 3:1

What religious obligations can be outsourced, and which must be personally performed?
לא יאמר אדם לחבירו העל את הפירות האלו לירושלם לחלק אלא אומר לו העלם שנאכלם ונשתם בירושלם אבל נותנים זה לזה מתנת חנם:
One should not say to his colleague, “take these [second-tithe] fruits to
Jerusalem for me, and keep a portion for yourself.” Rather, he may say, “bring them up and we will eat and drink them [together] inJerusalem .” However, they may always give [second-tithe fruits] to one another as gifts [inJerusalem ].
Comments :
The Israelite farmer was required to pay an elaborate series of gifts to the poor and the clergy (Kohen and Levite) before enjoying his produce. The “second tithe” was either produce or its cash value (plus 20 percent) that could be consumed only on pilgrimage to
Questions:
Why can’t a farmer pay an agent to take his produce to