I think it's just Riot not considering it a priority. The overwhelming majority of the playerbase knows how turrets, items, and abilities work, so improving the tutorial would have literally no impact on that majority. All of the people that used that sub-par tutorial eventually got to where they are now be it Diamond or higher, and the people in bronze aren't being held back based on what they learned in the tutorial, so the competitive side of the game isn't suffering either.
The current tutorial seems like an attempt to condense all of the basic mechanics into as short a time period as possible so that the player can then experiment on their own and not be bored with extensive tutorials. Couple that with the notion that actual experience is far more valuable than learned through hand-holding, then Riot has lots of incentive to get each user playing as quickly as possible.
If they did expand upon the tutorials, you have to figure what would be good tutorials to add and what would be too much. If you make a tutorial where you play each of the champion subclasses, then some people may be compelled to complete the tutorials before going into real matches, overwhelming themselves with the amount of knowledge there is to learn. While I see the benefit of providing a jungling tutorial, the fact that players won't have smite available to them until level 10 means that learning about the jungle items, uses for smite, and learning the timing of smite secures won't be applicable for a good number of games.
I want to say that if they did either add or remake the tutorials, it may be better to have certain tutorials unlock as you progress. Once you hit level 10 a jungling tutorial would become available to avoid the problems listed in the paragraph above. Perhaps at level 3 there could be a tutorial dedicated to understanding stats, explaining how they translate to abilities and overall gameplay, this would provide new players the freedom to experiment blind for a few games but the tutorial would still only be a light tap in the right direction. Maybe around level 15 there could be a tutorial dedicated to all of the various finer aspects like how turret damage ramps up and their other details, how trinkets and vision work, explaining the variances in cc, explain the differences in additive and multiplicative and diminishing return on stats.
convinced me that it was Attack Speed that made Teemo awesome.
i had no idea what to do then i found a turret and started to attack it, the turret killed me 2 times. at the '3rd back' i discovered i could buy items and my first one was (old) 
i thought my AS was too slow... every champ i played i used to rush pd to destroy turrets quick and i had no idea why i never got kills
