Drums, which are also known as membraphone are a part and parcel of the percussion family. Percussion are as old as mankind and have their roots in Africa (the cradle of mankind) even up to this very day they are still being played by the natives and the shamans. The moment when the first caveman could hit objects against each other to produce sound, be it hollow bones and coconut shells, percussive rhythm was born. When mankind evolved and acquired better tools, he then discovered that he could actually put a membrane over a hollow object to create an improved, more amplified and rounded sound - that's the first drum. Also note that, drums were used for various purposes, from long range communication to shamanic healing and spiritual invocations purposes, from dance to scaring away predators (today in South Africa, Botswana, Zimbabwe, Zambia, Namibia, Kenya farmers still use drums to chase away elephants from their farms).
Through the process of evolution, man also discovered that different drum sizes produce different sounds. This difference in pitch and tonality, is what mankind used in his long distance communications for "call and response" - which is basically a more organized and structured rhythm which is easier to understand. This drums and percussion were usually played by different people at the same time, hence the need to play and pause (talk and listen to what others are saying) - the Talking Drum.
That covers the prehistory part of the drum, now let's us focus on a drum as a modern day musical instrument. Fast forward to the 19th & 20th century, drums from the matches band were combined and arranged in such a way that they could be played simultaneously by one individual as opposed to a group of people. This did not throw away the ancient elements of, "call and response", "high and low", "light and dark", "loud and soft", etc this is exactly what the high snare and low kick drum are trying to emphasize when played in alternating manner to achieve contrast and rhythm.
We should also be cognizant of the fact that music is an art and like all arts it encompasses all of the art principles such as balance, emphasis, harmony, movement, pattern, proportion, repetition, rhythm, unity, and variety. Western music tends to adopt rhythms that are based on 4/4 time, which logically leaves us with either a kick or snare drum falling alternately on one of those beats. Other cultures have different time signatures and rhythms that they play on such as the Sangoma drumbeats in Southern Africa which tend to be polyrhythmic in nature and not straight ahead like its western counterparts.
To answer your question, if the snare and the kick were to be played in unison all the time, they wouldn't be able to create variety of contrasting rhythms, hence alternation is necessary. You can try it for yourself.