They probably do, but that doesn't mean it's necessarily something that could be set up in the sort of timescale seen this morning. TV-am would have had a few hours to arrange their alternative and still had access to their building.
GMB had 2 minutes notice for a 90 minute programme, by the time they all got to ITN and set up it might not have been worth it
Indeed, it was exceptional circumstances today, especially with it being a Bank Holiday so the show was shorter, fewer people were about to sort things out and make decisions and the audience would be lower anyway because people were having lie-ins. So in the end when it was clear they weren't coming back any time soon, they presumably decided it wasn't worth it. Be different if it was a normal day with a full-length show and presumably not a skeleton staff.
As you say, it was different for TVam because they were on air for three and a half hours on a normal weekday and there was a major news story, so they needed to get on air in some form. Plus TVam was a more complicated affair to deal with because it was all self-contained and not part of the ITV network, so if they weren't on air it would have been a blank screen, whereas today ITV continuity were able to take over and show repeats like they would every other time a show goes off air, so it's less important.