Not sure I can say why exactly, but we do this a lot. The Swedish present has a pretty wide extension. If it's clear from context that we're talking about the future and we don't want to stress some special nuance, the present is often the most common way of saying it. Jag ringer dig i morgon 'I'm calling you tomorrow/I will call you tomorrow' is one typical example of this. Probably the most natural way of saying it.