Dude. Why wouldn't they? Fall Out Boy has been around for years, but music develops, as do they, and even if they so much as tweak their music a small bit, they can still stay in the game! As this is more of an opinionated question, I will answer it in that form. In my opinion, Fall Out Boy is one of the greatest bands, and they still have what it takes to make great music.
Fall Out Boy was hugely popular when they first began releasing music. When they went on hiatus, it seemed like they were simply broken up for good. I think the reaction of former and new fans when they released their new video and announced they were together again shows they still have what it takes. All of their concerts sold out just moments after tickets went on sale, which is rare for bands of their musical genre. It seems like their fan base remains strong and they have new fans now as well and they seem very dedicated to their music.
I think Fall Out Boy can still be a great band, most of their fans are loyal and will be eager to hear new music. As long as they start putting out great music they will get great feedback. It is hard for most musicians to stay consistent but I think Fall Out Boy can try again.
Fall Out Boy had their time to shine and that time was way back in 2006. As it stands currently, they are simply old and outdated. When was the last time that anyone actually heard anything from the band in the news, or heard a new song of they performed. In short, they are toast.
Usually, reunion tours are all about nostalgia and milking money off of fans. It's probably the same with Fall Out Boy. Plus, while they were a decent band, they certainly were not pioneering enough for their getting back together being news worthy. Some bands get back together because their solo careers didn't go anywhere.
I doubt that Fall Out Boy still has what it takes. It's been a while since they were a group and they have probably lost the magic. These group comebacks and reunions are always forced and never live up to expectations. They're finished. They probably got back together because someone was broke.