a) IT usually block sites completely or reroute you to a warning page if there is a policy against visiting them
b) IT don't like being police so will only usually act if you are visiting porn sites. They like doing HR's job for them even less so don't tend to proactively pass on information.
c) People visit thousands of sites a day - identifying surfing for job vs. other surfing is not that easy. For example, if you visit timeonline.co.uk, is your primary purpose to look for a job or read the news? Who has time to do that level of analysis on internet usage.
d) The only time internet usage gets probed in any depth is after-the-event, e.g. if you are being warned or investigated. Even then, monitoring is primarily on a "live" basis. Due to being a pain in the ar*e to do, internet usage history only gets pulled out for fraud investigations.
e) Most importantly, nobody cares if you jobhunt on the internet, especially if you do your work and surf on your own time