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GodGinrai
I don't understand what you mean. They do communicate in their own way. Be it whether Yokoi is trying to stop him from messing around in class, getting caught up in his antics or her getting immersed in whatever game he is playing at the point.
We know Seki has a sister who tries to impress him, but he doesn't pay much attention to her, so Yokoi tries to have fun with her, play games with her etc. We are also aware that Seki's mother is a very talented but shy person, who messes up in things, but means the best and has entrusted Yokoi with stopping Seki from playing too much in class and thus gave her a box of rubber bands to disrupt his activities.
Also, Seki has friends in class who he plays with or talks to sometimes during breaks, if he isn't doing something else but that isn't a focus, because when that happens Yokoi is doing something else, typically involving a mess that Seki has made OR saving the robots. We have seen her go to Seki's house to return something she stole from him and have a small tea party with the sister.
Seki, has also been shown to be super-embarrassed about old games he played as a kid, which we saw during the school field trip thing, where Yokoi was trying to cover up for him.
Then there is the baseball boy and the girl who misunderstands Yokoi and Seki's relationship.
The interactions between Seki and Yokoi are there, they just don't have talking. Honestly, there is a lot of it, it just isn't overt, where they share words saying what they think of each other. Be it their teamwork at times or the more often clashing between them. You are left to see things for yourself, which is what a manga should do seeing as it is a visual medium.