Your question is rather about linguistics. Most reputable, academic works on this matter are, indeed, academic works, full of terminology and cross-references to other works. This makes it a bit hard to grasp. Let me try it in simple terms.
- Languages evolve
- This evolution is very similar to biological evolution (1) (2)
- This evolution occurs when people's generations change, so can't be managed by a single person
First of all, your question starts with a wrong assumption that today's words used to be the same from the beginning of the world. Take colour. It came from an Old-Latin colos, and that one, in turn, from Proto-Indo-European root *kel- ("to cover"). As you see, many changes in its orthography and pronunciation have already happened to this word. And many will occur in the future.
What drives the evolution of languages?
There are fundamental academic researches on this phenomenon. Wedel (2) suggests there are three major factors: (1) extinction, (2) blending inheritance, and (3) natural selection. Yang (1) also suggests an evolutionary model. What's interesting, Yang shows that both written and spoken parts of the language may evolve. So a simpler writing or pronunciation may trigger such change. Simplicity is not the only reason, however.
Say, there are two competing variants, colour and color. Today they co-exist. Maybe, in some professional areas a certain variant is preferred. One has better historical links, another one is simpler to write.
Note that besides two orthographically different competing variants, there are phonetically different ones. UK English: [ˈkʌl.ə] or [ˈkʌl.əɹ]; US English: [ˈkʌl.ɚ]. They co-exist as well, and we, language learners, occasionally use both.
It is very possible that, with time, one of the variants "wins" and another one extincts. But it is also possible they will co-exist for a long time. Or, maybe, a third competing variant arises, if some objective reasons appear.
Why can't we manage the process?
@Cerberus is right: People don't change the way they use the language. Some exceptions exist, but usually, this change takes place with a new generation acquiring the language. In other words, when our children study the language, they make what we call mistakes. If a certain mistake becomes more or less widespread, it triggers a change in the language.
If you are really curious, here are some fundamental works:
(1) Charles Yang's Knowledge and Learning in Natural Language
(2) Andrew Wedel's Exemplar models, evolution and language change