You can't. But you can request the feature.
It's my understanding that processing your voice into text is much harder, the more background noise there is. While your Macbook (or iMac, or whatever) has no control over noisy people sitting near to you, or a TV in the background etc, it does have control over the noises it emits, and as such does what it can to ensure optimal listening to the best of it's ability.
That's not to say it can't be done, but this is certainly how Apple have implemented it. You might want to look into 3rd party dictation software if the built in tools do not do what you require. You need to carefully consider the pros and cons of how important dicatation is to you, is it worth the hassle of it pausing your audio when you speak, or is paying upwards of $200 which is the cost of even basic 3rd party software value for your specific needs.
OSX, and indeed Apple products in general do a lot of things, especially for the price, but they are not in the business of making best of class apps free to users by bundling them in with the OS. They provided dictation, but you may want more, and 3rd party ecosystem delivers on that. They provide basic tools for notes, reminders, read-later software and so on, but non of these are as good as (for example) Things, or EverNote, or Instapaper and so on. They show you what you can do, it may be enough for most, if not see how other people can expand on the basic offering for improved features.
Think not "Can I do this with free Apple software without having to spend money to achieve my specific requirements", but "Can this be done, and what might I need to purchase to enable it"