Why does not God allow worshiping others except Him?

Verses 2-8 of Chapter 39:

Verily it is We Who have revealed the Book to thee in Truth: so serve Allah, offering Him sincere devotion. Is it not to Allah that sincere devotion is due? But those who take for protectors other than Allah (say): "We only serve them in order that they may bring us nearer to Allah." Truly Allah will judge between them in that wherein they differ. But Allah guides not such as are false and ungrateful. Had Allah wished to take to Himself a son, He could have chosen whom He pleased out of those whom He doth create: but Glory be to Him! (He is above such things.) He is Allah, the One, the Irresistible. He created the heavens and the earth in true (proportions): He makes the Night overlap the Day, and the Day overlap the Night: He has subjected the sun and the moon (to His law): Each one follows a course for a time appointed. Is not He the Exalted in Power - He Who forgives again and again? He created you (all) from a single person: then created, of like nature, his mate; and he sent down for you eight head of cattle in pairs: He makes you, in the wombs of your mothers, in stages, one after another, in three veils of darkness. such is Allah, your Lord and Cherisher: to Him belongs (all) dominion. There is no god but He: then how are ye turned away (from your true Centre)? If ye reject (Allah), Truly Allah hath no need of you; but He liketh not ingratitude from His servants: if ye are grateful, He is pleased with you. No bearer of burdens can bear the burden of another. In the end, to your Lord is your Return, when He will tell you the truth of all that ye did (in this life). for He knoweth well all that is in (men's) hearts. When some trouble toucheth man, he crieth unto his Lord, turning to Him in repentance: but when He bestoweth a favour upon him as from Himself, (man) doth forget what he cried and prayed for before, and he doth set up rivals unto Allah, thus misleading others from Allah's Path. Say, "Enjoy thy blasphemy for a little while: verily thou art (one) of the Companions of the Fire!"