That you can distribute something securely today, doesn't guarantee you can do it tomorrow - or next week or next year.
Also, your secure channel used to distribute the key may have limitations. Perhaps it depends on some person actually travelling between point A and B... Perhaps it's only available at certain times - e.g. weekends or during winter... Perhaps its capacity is very limited - maximum size per message and/or total messages that can be sent (e.g. it'll look suspicious if used too often with too much)... You may also want to "save" that channel for sending physical items (like OneTime-Pads or virus-samples), that actually can't be transmitted using radio or Internet - unlike text and images...
But the biggest obstacle is probably the time-aspect. Intelligence - the type spies gathers, and which are used in war and politics - has a very limited shelf-life. If you can't get the information out immediately, it looses its value - either because the slight head-start is gone, because whatever it warned about has happened, or because the same information trickles in from other sources.
So using the safe channels of communications - bringing it personally, couriers, letters past by officers on ship or planes, relaying the information through a chain of people - are probably all too slow. That leaves the fast - but unsafe and easily monitored - methods like radio, telegraph, telephone and the Internet. Through these insecure channels the information can be sent very quickly, but they're not safe... So you encrypt the information first, with the one-time pad you've gotten through the guaranteed secure channel.
And unlike your secure channel, it's almost 100% guaranteed that phones, radio and the Internet will be available also next year.