Does Mahayana Buddhism worship the Buddha as a God and, if so, why? The Buddha always held that he was a simple man, one among us, not a God. In fact, Buddhism is principally an agnostic philosophy. Then why does Mahayana Buddhism practice the worship of the Buddha?
No, the Buddhas are not Gods and Mahayana does not conceive of them as such. In fact, all forms of Buddhism - not just Mahayana - largely agree with the following:

However, let's be clear on the many ways in which Buddhism does not conceive of the Buddha:


The Buddha did not create the universe.
The Buddha did not create man.
The Buddha is not singular in the sense that many are described as Buddhas.
The Buddha was once an imperfect sentient being just like us.
The Buddha does not pass judgement and enact cosmic justice.
The Buddha does not cause the workings of karma.


Now, let's examine some of the ways Buddhism does conceive of Buddhas that seem to have some similarities with different definitions of "God":


Buddhas are perfected beings.
Buddhas have some forms of omniscience.
Buddhas have what appears to be some forms of supernatural powers.
Buddhas are the greatest of teachers.
Buddhas are worthy objects of refuge.


I think most if not all extant traditions of Buddhism would agree with the above and not just Mahayana.

Now, are Buddhas worthy of "worship?" Well, I don't know about that word, but I would say they are worthy of the highest respect and veneration. They have perfected themselves for the sole benefit of others. They have perfected all virtuous qualities. If you define "worship" as the highest respect and veneration, then I would say they are worthy.