I think that implies a sort of analogous relationship between verbal speech and spirit communication, which i do not believe exists. If you communicate with speech you lock yourself into that action and for the duration you can do little else (the limit here is your capability to multitask). If spirits talked like people talk I agree it could not happen 24/7, but there is no reason to believe they work like that.
What i think we speak about is telepathic communication. Notice I used the term communication and not speech. Humans communicate nonverbally all the time, whilst doing other things also. Spirits can do so also.
The problem is our existances, from our perspective, are radically different. Im going to ignore a lot of UPG i read here and say we have no reason to believe spirits have a language of any sorts similar to ours, or that they even use language and do not communicate by radically different means.
The human mind however is limited to what it understands, and finds ways to filter information given to it in forms that it can understand. As long as there is a connection between you and a spirit communication occurs. You can think about it like computers linked up to eachother downloading and uploading information to eachother, even if it is unimportant and simple information. Apply the right brain “program” to it and it comes out like speech.
Also just as we have an unconscious, spirits could also. Communication can take place in the unconscious mind for humans, so why not for spirits also? Sometimes when you “ring” a spirit, you get the “anwsering machine” instead of the full body, but an answering machine often times more than willing to give you some information.