At the end of the day, it's all a competition for a limited number of places. Your child is competing against everyone else's child. If you don't tutor, you're clearly reducing your child's chance of being admitted.
Different areas have different "admission" scores according to number of applicants against number of places. In some places, it's "super selective" because the number of places is proportionally very small compared with number of entrants, so the required "entry" score could be 90% plus.
In other areas, the opposite may apply and entry score could be more around 70%, i.e. the top, say, 150 pupils out of maybe only 500 entrants scored over 70%, so that's the default "pass mark".
Schools can't offer more places if they get a particularly "bright" group of entrants so the "pass" mark increases. Likewise, they need to fill their classes, so if they get a particularly "not so bright" group of entrants, the "pass mark" decreases to a level where enough are offered places to fill the class.
That's why it's not really useful to talk about grammars on a country wide level as different regions have very different demographics and therefore very different entry score levels.
If everyone is tutored, then inevitably the entry score increases, so that perpetuates the need for subsequent years to also be tutored hence how some areas become "super" selective simply due to high demand and the perceived necessity for tutoring. Likewise lower numbers are applying for entry, entry scores are going to be lower due to fewer applicants and lower entry requirements, and there's less pressure for tutoring and therefore the entry score remains relatively lower year on year.