There are numerous of reasons why/how such hand baggage gets through the net, and it a pain in the neck I agree! I spent 5 years on/at the counters in HKG and pax are a slippery bunch! There were checks at the immigration choke points and again at the gates, where a check-in agent was specifically assigned to monitor hand baggage as pax filed down for boarding
The last resort is ultimately the cabin crew though, as the item may have either gone unnoticed or it did fit the gauges. We used to add gate comments to those pax who may have told porkies about hand baggage, just to try and alleviate things. I recall one Nigerian trader who had a little wheelie bag which weighed 55kg! We'd seen it at check-in when he gestured to it on a trolley and it looked fine, but saw him straining to put it on the x-ray belt ;-) I've also seen a macintosh type overcoat filled with stuff so it looks like a lead coat !
A lot of times pax simply hide it or leave it with friends outside your field of view, or they hold up a tiny little thing and say "that's all". The worst offenders are those who have excess baggage and then disappear to "repack", but all they do is transfer it to their hand baggage.
Depending on the flight routing, there maybe hoardes of transit pax who have been through checked-in from the originating station. A/C type and booked-loads also plays a big part since they may have larger/emptier A/C and out stations allowed them to carry it.
It doesn't help much where manufacturers claim "airline accepted" on those little wheelie-bag things. What a pain it is trying to get someone to check it in when they bought it specifically to carry on.
Other factors are pax travelling from or who are used to the N.American piece system. This is a pain for dealing with hold baggage too. They quite often expect to be allowed to check-in AND carry umpteen pieces of bagagge and will not take "no" for an answer.
All in all I think it's a problem that will continue to face ground and aircrews alike, and there's no simple solution except maybe airlines providing "approved" bags and only allowing them aboard, but...
Best wishes.