Visa system was established and still exists in order to control international migration. Owing to their visa policies states can abridge the number of potential immigrants. So each migrant shall overcome the number of economic, administrative and many other obstacles before reaching the destination country.
Even for getting tourist visa the applicant shall provide a package of supporting documents to prove that the purpose of his stay in the country of destination is exactly tourism and the applicant has no intention to overstay the permitted period or immigrate illegally. If the purpose of visit is the permanent living, than the applicant shall get another type of visa and meet even stricter requirements.