@MrMcNasty the portal appeared and did not open for some days, leaving time to evacuate the population. And if I'm not wrong, it's said that the titans were roaming the city for some days before MC appeared. It's just a lazy plot device.
I wish that we got the NTR. It would've been more fun seeing the melt down at the comments than to read a 6.8/10 mediocre manga that everybody is going to forget after it's finished