Why can't nginx have more than 75 seconds proxy connect wait?

According to this doc here, nginx proxy_connect_timeout cannot have more than 75seconds delay.

I know its terrible to have an app with that delay, but I am being curious why this exact value?

From the nginx mailing list (see http://forum.nginx.org/read.php?2,214595,214597#msg-214597)

Connect timeout is limited by your OS's SYN retransmit count limit and timeout. Under FreeBSD it's 75s by default (3 retransmits with exponential backoff: 3s + 6s + 12s + 24s), under Linux it's looks like something about 20s by default. Looks like you are hitting this limit.

The only way to enlarge this limit is to tune OS, e.g. on Linux try adjusting net.ipv4.tcp_syn_retries sysctl.