I also don't like it when they act like they have to say it out loud to make it real when they have been macking on each other for presumably more than two months now.
It's a Japanese cultural thing. It's reality in fiction, and it's extremely common.
I've watched doramas where a guy and a girl have been living together for years, they have sex every day, they are in everything like a married couple... and then one day, bam, big conflict: she asks him to say loud and clear that he likes her!
That's right, he had never said it before! But she wants to hear it! Now!! She demands it!!!!
To which he reacts with utter shock and bewilderment, and totally panics! And when he eventually calms himself, he stutters something along the lines of: "It's difficult for a man to say that kinda stuff. You gotta give me a few more years to mentally prepare myself!"
Japanese culture is a culture of reading the context and reading the atmosphere, where you regularly imply things rather than stating them in an explicit way — and trust the other party to get your meaning without further clarification. In manga and tv shows it often leads to hilarious situations when someone tries to do this "imply not state" thing to a kuuki yomenai character who can't get a clue even if it's driven into their skull with a hammer.
Shiine and Ichika have a lot of this in their relationship.
They first kissed as a sort of test, out of curiosity.
Then one said that they should keep kissing every time they had a chance, and the other said okay.
Then, after a long time of this, one suggested that they should become friends, and the other said okay.
Then, again after a long time, it looks like Shiine is finally ready to say the words: "I love you!" Good for her! It took long but it didn't take years, lol.
It this seems extravagant to you, keep in mind that it's been only 10 regular chapters. In "Sekai de Ichiban Oppai ga Suki" Chiaki needed 42 chapters to drop formal address and finally start calling Hana by her personal name. And even now she still keeps denying that Hana is her romantic partner...