My (27F) bf (29M) doesn’t understand why I sometimes doesn’t reply to his texts.

My bf and i have been dating for a while now. We are even thinking of getting married soon.
When we started dating, i told him right away that i don’t like phone calls and that i prefer texting.
However, i am also not the best texter because i am easily distracted.
I can read a texting right now and forget to reply to it.
For example if he texts me while i am outside or on my way to work (walking,in the bus or train) i might take a long time to reply or not reply at all.
Or if he texts me two questions at the same time, it’s like my mind only focus on one question and reply to that one. Also it’s not like i don’t reply to important messages. But if he texts me something like “ have a nice day at work” and I don’t respond he gets frustrated. He always takes it personally and the thing is i don’t only do it to him. I feel like it is not my fault for being easily distracted and you should understand that.