The "why" answer is because when Microsoft adds features they would like to be paid for them.

The idea behind paying once for a perpetual license is you get what you pay for and do not have to keep paying.

The idea of the subscription service is that you are paying a little bit each month or year forever and when they add something new, you get it.

There is a commercial program "Dragon Naturally Speaking" which includes an Add-In for Office. Its features are superior to those offered as a part of Office or Word but it is not cheap.

You can also use Word Online, which is free and includes both dictation and transcription service. However, it requires a (free) Microsoft OneDrive account and you have to be online to use it. It creates, edits, and saves Word documents on OneDrive. It is not the same as the desktop app. Differences between using a document in the browser and in Word - Word