Actually they did. They improved the graphics, increased, the size of the map, added some things that were not there in IV, hunting, heists, etc.
As for the short campaign, 70 missions is actually good. There are open world games with far shorter storylines.
As for the terrible GTA:O launch, Rockstar themselves said it would be a problem with lots of people logging in at once. It became smooth after a couple of weeks. Loads of people crashing into servers is something beyond their control.
However, GTA IV was indeed tougher to make. Considering it was their first HD game, they did do really well to make the game feel good.
Again: These improvements are not denied by me. I say that they don't feel like 4 years worth of improvements. After all they were improving the engine with: TLAD, TBOGT, Max Payne 3, and Red Dead and those updates both visually and in game design were used to help make GTA V. There was a heist in GTA IV so I don't know why you're including that in the list of things they added, and they also removed quite a bit of gameplay from GTA IV. The basic design feels more sloppy and many of the side activities feel worse than they ought.
Hunting is nice but it's stupid that only one character can do it, would be improved by an inventory (which red dead had so the work for an inventory had been done in the RAGE engine), could have had more animals from Red Dead, and potentially could have just been done on the fly like Red Dead so we could count any animal kill as a hunting kill.
Bounties are a joke with only 4. GTA IV had vigilante with 30 Most Wanted to kill. They couldn't even make half of that for bounties to capture or kill? Why don't we have to tie any of them up or take them by force? They all eventually come quietly without any form of restraint. This is the quality of 4 years of improvements?
Rampages are nice but there are too few. (I think 5 but I forget the exact). GTA III had 20 rampages. Why does GTA V have less? They haven't been in the main series since Vice CIty and their return is rather weak.
Random events have seen improvements but feel less dynamic than Red Dead's where they were truly improved first. GTA V only adopts them and puts them in fixed locations... hooray?
Properties feels extremely lackluster. One I suspect was always going to be part of the story in some way (the airstrip that Trevor can buy) because it features a cutscene. The rest feature phone calls or text messages. The activities they ask you to do are generally very simple and repetitive and they call extremely often. It feels like they rushed it out because they realized people wanted the feature.
The campaign is the shortest in GTA since Vice City. (Or the expansions if you count those). Even the PSP games had longer campaigns or similarly length campaigns. Having missions like "Buy masks" is ridiculous. It's not really bad for it to be short, but the length of the campaign does hinder some of the story and more story could have given us more awesome gameplay. Imagine having more missions for Trevor's TP industries or having to deal with the repercussions of leaving the gang life for Franklin. These things could have lead to awesome missions but we only really explore Michael and Trevor's past.
GTA: O dealt with a lot more than trouble connecting. There were lost characters, lost money, lost cars even after patches trying to fix these things. It is now stable to play but it took a bit long to get to this point and we still don't have all of the features.
All of it leads up to a game that excels in many areas but falls short in others. It's a weird mix of quality and a lack of quality that feels uncharacteristic of Rockstar. I kinda suspect the game was started later than we believe and was rushed to completion to get out before people dumped their 360/PS3 for the next-gen systems. I doubt highly they just put less effort into either game. They both probably had severely different circumstances for development.
Yet they can't even do something as simple as make a police helicopter spawn on a police station helipad (in single player), they can't even do something as simple as make a hearse spawn at the cemetery?
They probably could have done it but didn't for reasons unknown. What's really important is why you think this is so important. This isn't a major feature. You're talking about a game that overhauled every basic gameplay feature and are acting like only this matters. Was it something they needed to improve? Yes. Did they improve it in V? Yes. Does it mean that IV had no effort and V a ton of effort? Hell no. Both had a lot of effort put into them and they both have areas where they succeed and fell short.