Why did the present participle become more popular than a regular active verb?

I've been studying Latin by myself as a kid in middle school, and I've gotten fairly advanced with it. However, in Latin and most other languages, the present participle is/was almost never used in the place of an active. In English, this isn't the case. For some reason, it's considerably more common to announce "I'm writing," rather than "I write."

Often, I wonder exactly how the present participle came to be as ubiquitous as it is with the verb "to be," but not as a regular adjective. What is the reason for this odd choice? Why has become so frequent?

John McWhorter writes that it "is almost certainly" due to the influence of Celtic languages spken alongside English (1):

Given that Celtic languages were right there alongside English all the time, if English and Celtic share features of grammar that are rather unusual worldwide, then obviously Attention Must Be Paid to Celtic. Another Celtic inheritance is almost certainly the progressive -ING one: in any other Germanic language, where I say I AM WRITING they would just say I WRITE. Again, Celtic languages have always done it as English now does.