why the game don't have daily login
It does. You get a daily crystal and 4 hour crystals... And an alliance crystal easily if you send help to your alliance
He's referring to MCOC's (and many other games') log in calendar rewards. I imagine it doesn't have it at this stage as they don't think it's necessary to keep people logging in or it's not yet developed.
It does not. A daily login bonus is something that rewards you based on the number of days you've logged in, sometimes consecutively, sometimes not, to encourage people to play more frequently. They usually include a variety of item rewards and sometimes increase with value or have value items mixed in periodically, to encourage more logins.
Here are some pictures to help you understand what it means.
In general, they're not a bad idea. However, the game has a lot of incentives to log in currently. There are constant events including 6 hour blitz events and 1 day events. These types of log in incentives focus on and encourage the group alliance aspect of the game, which is probably a better direction for the game to go in than a daily log in. There is always space for a daily log in though, it just adds more free stuff to the game.