Why does this guy only reply once a day even though he leaves me on read the day before?

I'm a YouTuber who reads books for people and sometimes I would like to check what other YouTubers with the same activity as me read. I accidentally stumbled upon a guy's channel and he doesn't show his face or has any SNS attached to his youtube channel. He was asking for book suggestions though and said that any suggestion could be sent via EMAIL; so I emailed him with some book suggestions and we somehow agreed to be friends. We were emailing each other once a day and I was cool with it thinking about the old times where people would just email each other and no texting at all. and then about two months passed and I would still get one reply per day. I thought maybe it's time to ask for a private SNS so that we can be more comfortable in getting to know each other so I did but he refused my request saying that he doesn't want to expose his identity (pics etc) and that email is fine for him so I respected that. We talked for another two weeks but he seemed to bit too conservative about his life (although he opened up about his personal feelings very few times). for example, I asked when is his birthday and he only replied with the month lol (like is it too hard to specify the day as well?)

and yesterday I figured out that he reads my email immediately after I send it but for some reason, he only replies after a day. He has a job and I understand that he could be busy working but can someone really have time to read a text but not 1mn to reply back?

I'm not really looking to date this person but rather I want to be friends with him but it kinda feels like after talking for more than 2 months we're still strangers.

I'm worrying if I might be bothering him and he's just being nice.
Why does this guy only reply once a day even though he leaves me on read the day before?
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