scared driver delight,
Joseph has it right. It’s intuitive.
While you are correct that our day starts in the evening, with ma’ariv as the first tefilla of the day, ma’ariv usually follows closely after mincha. So although mincha technically is the final tefilla of the day and ma’ariv the first of the next day, it makes it easier for us to find ma’ariv if it’s right after mincha in the siddur. Shacharis comes first in the siddur, because that’s the first tefilla you do after a long break (sleeping), when you have probably put the siddur away and then taken it back out. Again, it makes it easier to find.