This is due to the fact that English has evolved becoming more and more an Isolating Language (it's not the only example, but it stands out as a peculiar example, considering its classification).
Isolating languages — often contrasted with Synthetic Languages — are characterized by the fact that their morphology is not that considerable and sometimes not existing at all. What is intended by lack of morphology? By saying that, we mean that such languages have a low morpheme-per-word ratio, so their words are hardly "divisible" in smaller elements, which are called morphemes. To understand how morphemes work, check out this answer I posted.
Summarizing what we need from that answer, English has a famous example for morpheme, which is the -s for the third person singular in verbs. It's only a letter, something "very small", but it tells you something. And that's what a morpheme is: "the smallest element in a word or in a statement that carries meaning and that cannot be further split (into smaller parts)"; the important part from this definition is "carries meaning".
English, especially if compared to other languages, has this low ratio, so it needs other means to disambiguate. You can see this simply by thinking about the fact that English almost always requires personal pronouns to come before verbs. Take this example:
- We speak
We can't tell if the first example is the infinitive, first person singular, a noun, etc. While we know that the second one is the first person plural of the verb "to speak" because of the personal pronoun being there. Note that "speak" is itself a morpheme (it's the root that carries the lexical meaning), so it's not that English lacks morphemes completely, it just has less than other languages; it isn't totally isolating yet.
"Speak" is an unbound morpheme, which means it can stand by it self. Bound morphemes can't, so they need to be attached to other words. Roots are categorized as unbound morphemes; endings, prefixes, etc. are categorized as bound morphemes.
Let's make a contrastive example using Italian because it is good language for explaining how synthetic languages work:
The first one is unambiguously the first person singular of the verb "to speak" in Italian, as much as the second is the first person plural. But as you can see, neither of them is preceded by the personal pronoun, which is optional in Italian. (Personal pronouns are omitted in Italian unless the context is ambiguous or used to express contrast, something like "I said X, while you said Y.")
Another famous isolating language is Chinese, which completely lacks morphology: no conjugations, no endings, no number indication (singular vs plural). When a language lacks morphology, word order is one of the means adopted by the language to compensate for this loss.