Why do Jehovah’s witnesses call on the homes of people who already have a religion of their own?

Jesus instructed his disciples to “search out” deserving ones. (Matt. 10:11)

Likewise today, our objective is to reach everyone possible with the Kingdom message. This means imitating Jesus and his apostles in their approach to the disciple-making work as well as keeping abreast of the changing times and the varying circumstances of the people in our territory. (1 Cor. 7:31) Street witnessing has proved effective in many countries, as has witnessing in parks, parking lots, or wherever people can be found. We are commanded to preach the good news by Jesus Christ.

It is a command from Jesus.  Matthew 28:19, 20-“Go, therefore, and make disciples of people of all the nations, baptizing them in the name of the Father and of the Son and of the holy spirit, teaching them to observe all the things I have commanded you. And look! I am with you all the days until the conclusion of the system of things.”

Spiritual harassment and against the law. Because their harming your conscience 

they need mor people so they can get More donations

Misguided delusion that they are anointed by God.

It's a pyramid scheme, like selling Tupperware. You get extra credit for not only buying their stuff but getting other people to buy it, too.

“From House to House.” Jesus went right to the people with the Kingdom message, teaching them publicly and in their homes. (Mt 5:1; 9:10, 28, 35) When he sent out his early disciples to preach, he directed them: “Into whatever city or village you enter, search out who in it is deserving.” (Mt 10:7, 11-14) Such ‘searching out’ would reasonably include going to the people’s homes, where “deserving” persons would heed the message and the disciples would find lodging for the night.—Lu 9:1-6.

On a later occasion Jesus “designated seventy others and sent them forth by twos in advance of him into every city and place to which he himself was going to come.” These were not just to preach in public places but were also to contact people at their homes. Jesus instructed them: “Wherever you enter into a house say first, ‘May this house have peace.’”—Lu 10:1-7.

In the days following Pentecost 33 C.E., Jesus’ disciples continued bringing the good news right to the homes of the people. Though ordered to “stop speaking,” the inspired record says that “every day in the temple and from house to house they continued without letup teaching and declaring the good news about the Christ, Jesus.” (Ac 5:40-42; compare Dy, NIV.) The expression “from house to house” translates the Greek katʼ oiʹkon, literally, “according to house”; the sense of the Greek preposition ka·taʹ is distributive (“from house to house”) and not merely adverbial (‘at home’). (See NW ftn.) This method of reaching people—going directly to their homes—brought outstanding results. “The number of the disciples kept multiplying in Jerusalem very much.”—Ac 6:7; compare 4:16, 17 and 5:28.

The apostle Paul told the elders of Ephesus: “From the first day that I stepped into the district of Asia . . . I did not hold back from telling you any of the things that were profitable nor from teaching you publicly and from house to house. But I thoroughly bore witness both to Jews and to Greeks about repentance toward God and faith in our Lord Jesus.” (Ac 20:18-21; compare KJ, Dy, AS, RS, Mo, NIV, La.) Paul was here speaking of his efforts to preach to these men when they were yet unbelievers, persons needing to know “about repentance toward God and faith in our Lord Jesus.” Thus, from the start of his missionary service in Asia, Paul searched “from house to house” for spiritually inclined persons. Finding such ones, he doubtless returned to their homes to teach them further and, as these became believers, to strengthen them in the faith. Dr. A. T. Robertson, in his book Word Pictures in the New Testament, comments as follows on Acts 20:20: “By (according to) houses. It is worth noting that this greatest of all preachers preached from house to house and did not make his visits mere social calls.”—1930, Vol. III, pp. 349, 350

For more information JW.ORG

They want to convert people.

Thats what witnesses of every type do. They witness for their beliefs and experiences.

God calls upon all Christians to witness the Holy Bible Gospel to everyone, not the JW NWT.

Acts 2:32 KJV — This Jesus hath God raised up, whereof we all are witnesses.

Because their cult leaders tell them to do so.