It’s almost a cliche in war movies for a soldier to climb on top of an enemy tank, open the hatch and toss a grenade in.
My question is this: I assume the hatch has a locking mechanism of some type. Are/were hatches usually locked to prevent the above scenario? If not, then why?
It seems odd that they wouldn’t be locked. Maybe it’s so the crew can evacuate easily?
Or maybe it’s so the filmmakers can show a heroic lone infantryman taking out a tank single-handedly.
I ask because I just watched a great movie called Lebanon and a Lebanese major would often open the hatch and drop into the tank to discuss the situation.
Not being a tanker, I can’t tell you with great certainty but it seems that leaving it open would help with:
Turning out quickly which helps you get more visibility and use the gun on top.
Evacuate quickly if you’re hit.
The downside is unlikely to be all that important. Tanks are often accompanied by friendly infantry which could easily see and shoot anyone who tried that. Tanks might not be so accompanied over open ground but then, the chances of getting on top of a tank in open ground are slim.
Even in dense terrain (forest, urban) without friendly infantry, I don’t think the scenario is likely. If I were infantry fighting a tank, I’d bet on RPGs and IEDs.
Tank hatches lock open and lock shut. Travel on rough terrain demands such an arrangement.
Rule of Cool trumps real-life every time.
Then how do the tankers get in?
Through the barrel. 
No, they use the emergency exit at the back, and then go up front to where the lever is to open the front door.
Or is that school bus drivers?
It’s a manual lock. You have to be inside the tank/APC to en-gauge the lock.