Well the real reason is that this is either how long it takes to load the level, or how long the devs think it'll take.
I suppose they could concoct something in lore, but that's the truth of it. Another good question - why is the tunnel always, always, always identical? - I can be flying totally different distances it totally different parts of the Galaxy in totally different ships. It always looks the same, weirdly. Don't get me wrong, it's fine. But it doesn't make much sense when you stop to think about it!
I have a funny feeling that early in the dev cycle it was interactive like an interdiction mini game with the possibility of dropping out at the wrong place if you didn't fly well enough. But this was negatively received and by testers and taken out to be replaced with what we have today (basically a loading screen, I agree).