Why isnt a random port chosen for SEED node sockets?
All nodes are listening on port 18333 for inbound connections.  Any inbound connection will use this port.  So addrlocal will show localIP:18333.  

Any outbound connection will use a random port so addrlocal will show localIP:xxxxx

So every connection is either random &lt;----> 18333  OR 18333 &lt;----> random.  The node that initiated the connection (out) will always use a random port.

This is how all tcp/ip web services with known ports work.  Web servers listen on port 80 or port 443, browsers use a random local port to connect to them. 

Edit: Should note that port 18333 is for testnet nodes. Would be 8333 for main net.  Also if on linux netstat -ntlp or netstat -ntp will show all tcp connections