Because networking is difficult with a lot of unknowns, P2P is a resource hog with many security issues, and encryption in itself is a beast.
A lot of the video game studios would love to use P2P instead of having to invest in infrastructure that cost money. Destiny is a example why they shouldn't. Destiny P2P system shows on average that anything over 5 users causes major lag in data transfer. Users home networks are equip for handling data coming to them, but not for data being pushed out. Same thing for cell phones they are design for small burst of data going out not constant data being uploaded. P2P is to much a resource hog when it becomes a time sensitive data transfer.
The other problem Destiny prove to be a issue with P2P is a small security issue with ddosing. You beating someone in a video game well a instant ddos and bam lag spikes. Lag spike in a P2P system are murder because they are instantly felt by all.
Another unknown issue with networking is routing you never know how or where your packets are being sent. If one user packets are for some reason being routed through Uganda then every user might have to send the same package 3 or 4 times each time till it reaches them. You would be surprise how many packet drops happen between isp and networks. This is even a bigger issue with P2P because a packet drop is multiple by the amount of users.
And finally we get to the fun world of encryption. It is fun because it is simple to say all the terms that make it sound encrypted and secure but it is a iceberg of hell and dog poop. Creating secure channels while creating encrypted message while insuring that the end party are a trusted end users while ensuring all the algorithms used are up to date and solid is horribly boring and tedious. Did you know that if you compress something then encrypted then you just broke your encryption? That is because compression algorithm has a repeating pattern that makes it easier to break encryption. It is little things like that and more why you need a whole security team of red and blue team when you ever use the word encrypted.
Also to the other problem you eluded to cross platform. Making anything cross platform is it's own major heart break issue. When you toss in phones as well (I run in corner screaming and crying thinking about it). You might say something like java can handle it all, but it doesn't. You still need to test everything over and over again. Every computer can be running a different version of the Java virtual machine each having its own different bugs. Then you get to the realm of cell phones on top of creating a desktop app, and I would just walk away. I would charge someone billions to handle that kind of dev work.
The situation isn't pathetic it is a iceberg problem. The amount of complex problems that come with creating a stable P2P cross platform encrypted solution that is easy for end user to use, and doesn't compromise in certain areas would cost millions of dollars upfront to build.