Because, in Lev 27:30 it says, it belongs to God, it’s not yours nor mine. It is the property of God. For this very reason, if you sanctify tithing, we ought therefore not to collect it anyhow unless we are permitted to do so. If you are an owner of something, you wouldn’t want someone who you did not authorize collecting it, right?
Having said all this: The owner of tithe says;
“Behold, I have given the children of Levi all the tithes in Israel as an inheritance in return for the work which they perform, the work of the tabernacle of meeting.” Numbers 18:21 NKJV
So now the question is, are you one of the levites hence, you are collecting tithes as according Lev 27:30 or Matt 23:23 or Mal 3:10? If not, show biblical proof that non-levites like yours were already authorized by God to collect what belongs to God?
Should your answer be the verse:
“Here mortal men receive tithes, but there he receives them, of whom it is witnessed that he lives.” Hebrews 7:8 NKJV
Knew that mortal man in the above verse in the right context still refers to the levitical priest but, should you insist that it refers to mortal men not just Levites. Well, the verse did not really elaborate whose who are the mortal men as referred in the verse, apparently, all men are mortal and without adding or subtracting on the verse above since all men are mortal, do you mean to say that all men on earth should receive tithe? Who shall be the giver if basically, all are a receiver?
Should you cite Melchizedek (non-levites) as someone who collected without proof of authorization from God. Well, is your priesthood the same as Melchizedek? If yes, do you mean to say that since Jesus priesthood is in the order of Melchizedek, are you saying that your priesthood to is in the same order? So, You, Melchizedek and Jesus have the same order of Priesthood? Was during the time of Abraham, tithe was collected by someone whose priesthood lower than Melchizedek?
Note: Pardon me for wrong spelling and/or grammar.