Your astonishment at the fact that bayonets are still used is perfectly reasonable, for I think they are a useless anachronism.
NOTE: I am in a very much outspoken minority when it comes to this subject, and no doubt other military dopers will disagree with me.
Modern rifles are much shorter than their WW1 counterparts, and that means less reach. They are also somewhat less able to withstand the kind of abuse that bayonet fighting will put on the rifle and still function. Modern rifles also tend to use knife bayonets, which are more useful as knives but less effective than the old spike bayonets. The prevalence of electronic and optical devices on modern rifles such as flashlights, lasers, supressors and optical sights also make regular use of the bayonet inpractical.
In the infantry sections/squads of most Western armies, only a minority of soldiers will be carrying bayonet equipped rifles. The prevalence of light machine guns (generally no bayonet mounts), underbarrel grenade launchers (which generally preclude the use of bayonets), and designated marksmen (whose rifles generally cannot mount bayonets) means that only 20-50% of infantrymen will have them.
The British army is famous for having launched a “Bayonet Charge” in the early days of the Iraq war, to great effect and supposedly validating the usefulness of the device. When I raise the obvious question of how a standard 8 man British army section, with 2x Minimi light machine guns (no bayonet mounts), 2x L86 LSWs (no bayonet mounts) and 2x AG36 equipped rifles (you guessed it, no bayonet mounts), is supposed to have done so much damage with their bayonets, no one has ever given me a straight answer. I suspect that while one or two people may have been bayoneted, the battle was probably won the same way most other British victories over third world armies were; by being better shots and having more machine guns.
A pistol is a much more useful secondary weapon to have than a bayonet.
There are certain specific uses for bayonets, it’s a good, cheap crowd control weapon, and can be used as such as a last resort, but there are better crowd control devides in the tool box. It is a nice way to build confidence and aggression for new soldiers, and the training is fun and relaxing. That’s about it.
No known version of the FN SCAR feature a bayonet mount.
I think that the weight of the bayonet can be substituted with an extra rifle magazine, a far more useful item, and the time used on bayonet training can be used for more productive ends.