Oxidation
People will describe what has happened many way but it's simple oxidation of the metals due to moisture and corrosive chemicals in the soap from pressure washers. Both the bolt and aluminum has oxidized, bonding them together like a glue. The portion of the bolt that extends past the threaded aluminum section into the body of the swingarm is rusted (oxidized) also. All of this is fighting against you. How bad do you want to fix it. I've had good luck drilling 2 or 3 small holes in the swingarm (less than 1/8th inch), inline with the bolt (on the backside of the swingarm - wheel side ). Drill down until you hit the bolt. Now pour whatever your favorite penetrating oil is, into the holes and let it set over night. In the morning if the oil has gone away, pour more in and let it set over night. Now try to take the bolt out. If it's still turning so hard that you think it will break off, it's time to use some heat. You will need to get the aluminum up to 375 or 400 Deg. F. Normally this will require the use of an oxygen-acetylene torch. You're trying to heat the aluminum fast enough, so you don't put a lot of heat into the bolt. The aluminum will expand, releasing it's grip on the bolt. Heat the general area of the bolt location from the back side (wheel side), moving the torch around so you don't have any localized heating damage. Use a kitchen thermometer to check the metal temperature. When it's up to temperature start taking the bolt out. The rust on the threads of the bolt that extended into the swingarm body will be your real problem. The rust could still damage the aluminum threads. If the bolt starts to turn hard you can pour some oil into the holes you drilled, and yes it will smoke as it is heated by the hot aluminum. Or you can try heating the aluminum more. Once you get the bolt out, run the correct tap down the hole, to clean up the threads. If you've damaged the threads beyond salvage, it's time to put in a hele-coil. Clean the oil out of the holes you drilled and plug them with a sealing product. A little note here, silicone products draw moisture when they cure. Be sure and use plenty of grease on the bolt and put grease in the hole before putting a new bolt back in. I know it sounds like a lot of work but it's worked for me and I haven't had to hele-coil any yet.