I suppose because you feel like you exist- right? If you stub your toe on a table leg- it hurts. If a family member stubs their toe on a table leg- they will likely react in a manner that suggests that they are in pain. So- if people's reactions to things- not just painful things- nice things too. Tasty food, funny jokes is similar to how you would react, it seems likely they are experiencing life in a similar way to you. Even if it isn't real, it still hurts- presumbly.
Has anyone ever made a meal for you? Did it taste real? Did it put on weight or, make you shit eventually? So- the meal was real presumably? Presumably, so was the person who cooked it. So was the farmer who grew it, the shop assistant who sold it. Unless you're not real too. In which case, we're all sharing this illusion together.
If the principle is- you can't hurt people because, they're not real- surely it must work in reverse too. If someone you know suicides or hits you, will it affect you or not? Maybe you can convince yourself that they aren't real so, it shouldn't. Still, you must have witnessed other people feeling upset about things, so- whether they are real and whether the thing they're getting upset about is real, it still affects them in a similar way to how things affect you- presumably?
I actually sometimes wonder if people who truly believe this isn't real have some advantage in some way. Can you not somehow break the rules and be free? Choose to be happy because, nothing is actually real. None of the bad stuff is real. You don't need to comply to any of it.
I suppose I'm convinced of reality because there are real life consequences to not eating, not drinking, not working, breaking the law, injuring ourselves, injuring others. Plus, everyone in life is reacting in similar ways to me.
I suppose unless we have the power to escape certain physical realities and consequences for doing certain things. In which case- we can't escape this state- it's the reality we're stuck with regardless- whether everything exists or not. So ultimately- does it even help to believe it isn't real?