the big reason is the following two
1-No team practice tool mode has been made yet
2-Only korea has the structure to form proper training teams
1-No team practice tool
In basketball, football, soccer, all sports, if a team wants to practice basic strategies or combos they will go onto the court and practice the situation out at least 50 times, however many times it takes them to get it right. For example let's say my basketball team wants to practice this strategy where I pass the ball in a certain way to player X, and X dribbles in a certain style to then make the shot. This is something I could easily practice and duplicate in a real life world. However in a virtual world, since no team practice mode has been created yet, this is impossible to do. This is a HUGE problem for all league teams, and I hope riot develops a proper practice tool meant for pro players and teams in the future
2-korea and training teams
The big reason pro teams practice among other pro teams, even if it means revealing strategical secrets, is because they have no choice. The average basketball team has like 40 something players, along with a training squad. Because of this they can constantly practice games against each other for hours upon hours. They are playing against high tier players at the highest level in their field and are getting quality practice. They don't need to "scrim" other teams, for they already have a team they can scrim against on their own court.
However very few teams can afford to have multiple training partners under their banner, and even when they do, it's very low quality practice. The difference between league and the NBA. is that the NBA has the top 200 or something players competing against eachother. while the NA lcs only has the top 50. MEANING that a NA LCS league player, is most likely better at league, than a pro basketball player is at basketball. (sorry if that sounds confusing) the standards to play in the NA LCS are much higher than they are for a basketball player to play in the NBA. In the NA LCS you can get fired simply for not being the best, not because you were bad, but simply because you weren't near perfect. Sounds pretty harsh right?
So even if a LCS team had a practice/training squad that they could scrim against, chances are, they would be much lower qualtiy then a LCS team, thus making low quality practice.
Just look at splyce, by forming an alliance with G2 in season 6 summer split, they went from a last place team, to a 3rd place team. Quality practice is important and it helps by a lot.
The reason top teams scrim so much is simply because their is no effective alternative ways to practice, vs real life sports where they have a huge list of ways where they can practice.