That's because layer 5 and 6 don't really make sense in the TCP/IP stack. Maybe you could say TLS is one of those layers, it is definitely a layer between tcp and http, and in haproxy documentation it is layer 6, but it also doesn't map to the OSI concept for those layers, and is often said to be layer 7 as well.

And then there is quic, which is a transport protocol, so kinda layer 4, but it is higher than udp, but it also has TLS built into it.