Because the rules explicitly state that it is not an Extended test. I assume it is because they want to prevent players to find every information by just accumulating time. But if you want to houserule this, as GM it's your call.
Same goes for Trying Again. I assume the very reason this rule exists is to ensure that it is not impossible to hide. If the IC can just roll and roll and roll again, even the best hacker has no chance at all to escape detection. "Attempted shots, sword swings and punches" are explicitly excepted from this rule, as it wouldn't make sense. I think, it makes a lot of sense to implore this rule on all kind of detection tests. Otherwise, every attempt of sneaking is futile. Again, if this is your call as GM, I accept it that way. Although I have to admit I find it frustrating that then the last possibility to beat patrol IC is closed.
So, did I get the situation right. I lost 30 seconds with my first failed attempt. Then it takes me another 7.5 seconds till I find the file I'm looking for. So I definitely have the location. Half a combat round later, the patrol IC detects me and launches the probe IC, right? That means, that I have one-and-a half combat round to download the file. Is that correct?
If so, I check the file for protection and/or databombs. If something in the situation is different, please clarify.
Sorry to keep this. I'm still a bit unsecure about both the official matrix rules and your houserules, but I give my best to keep up.