It's an unofficial policy for most drive-thru restaurants to prioritize drive thru orders above inside store orders.
The reason is that typically, drive thru does more business than counter orders (around here, it's 60% drive thru 40% counter orders).
At our store, due to how the drive thru is situated, it isn't necessarily faster to go drive thru. There are two lanes that converge into one, and when the cars converge is when it starts getting jammed.
People will try to speed ahead of other cars to make sure that THEY get first, and if it gets out of order (orders store in our system in the order they are started), then depending on the cashier they can get easily confused, which causes chaos at the windows.
More confusion occurs if the orders switch while the runner is bagging one order when another order, that was behind the one they're working on, suddenly is in front.
And even more occurs when there is a multi order, since they are stored kind of funky and don't bump themselves to the front of the line, which means we have to watch at the bottom to see what is paid out rather than what order they're stored in (don't ask me why the system was designed like that).
So long story short, our store's drive-thru design is horrid at times and so it makes it worth it to go into the store.