Because you did something wrong. Try to open "cmd" on windows and have a ping test to ps4's IP address. Both PC and PS4 have to be the same sub network.
If you don't have any router and you're going to connect directly from PC to PS4 with an ethernet cable, here may help you:
- Set manually on your PC (Ethernet):
IP: 192.168.1.2
Subnet mask: 255.255.255.0
Default Gateway: 192.168.1.1
DNS: 192.168.1.1
- Set manually on your PS4 (LAN):
IP...