LAN games can't even saturate gigabit. Cat6 support 10gbps as long as it is within 100ft. Plenty for him.
There are very few use cases for multi gig network at home. 4K Blu-ray maxes out at 128mbps.
Because if you are going through the trouble to run the cables, the bulk of the cost is the labor to make the runs, not the price of the cable. You are also putting something that is effectively permanent into the home. So why in the world would you not put in within reasonableness, the more advanced options that support technologies that already exist and use cases that already exist? Again, sure you don't have even gigabit right now, but this cable isn't just for right now, it is for the next 10-20 years (when was the last time you replaced the electrical cable in your home, or the phone cables, or the co-axle cables)? So again, in 4 years from now when 10 gigabit and faster internet feeds to the home are industry standard, you don't want to be replacing this cable install because you decided to not put in CAT6a now and saved something like $90 total on a $1000-2000 project and limited yourself to something 10 times worse. Great thinking there.
Again, I get it, and this is why I don't recommend CAT7 or CAT8, because there is nothing currently out that requires CAT7 or CAT8. In the future it is "possible" that CAT7 or CAT8 might be able to do 40 gigabit over certain distances, but nothing has been certified for that speed over any distance, which means all the current CAT7 or CAT8 might require a revision in order to support it in the future when such tech becomes available (which means you would need to replace the line anyway and gained no real benefit over what CAT6a provides now). CAT6a gives you 10 gigabit over 100 meters. There are VERY few homes that can not be wired with CAT6a and exceed the distance rating for 10gbps service. A 1000ft spool plenum rated of CAT6a will run you $250-300. A 1000ft spool of plenum rated CAT5E will run you $180-210. The labor the run the cable is the same, approx 60-90 minutes per cable in a retrofit depending on distance and accessibility (could be longer if firewalls are in the home, or brick/concrete block or lack of access to crawlspace/basement/attic). If contracted out that would run ~$100-$200 per hour of labor depending on market/location (obviously doing it yourself would save on that cost, but you would still need to spend that time and your time is also worth something as it is time you are not spending with your family, or doing other work). So again, the real cost isn't the cost of the wire, it is the labor cost as you can see just 2-3 runs will cost more in labor than the spool of cable.