oy: A Rebbe of mine once told the story that he found out a Talmid was learning in the bathroom. The Talmid’s logic was very simple. If more learning=more Olam Haba, then why should he lose out just because he has to go to the bathroom.
The answer is also very simple. Torah is about much more (and much less) than gaining knowledge or even gaining an understanding of HKBH. It is because Hashem wants us to, which is both the simplest and most complex reason imaginable. Hashem doesn’t want us learning or checking our email during Davening. I would not be so brazen as to guess which is worse. I could definitely hear that learning during an Assur time is both a Bizayon to the learning and the Davening, though, which would make it worse than checking email, which is just a Bizayon to Davening.