Consider if a new player got help from 30 people instead of 3 each day.
Neighborly help is not all that helpful for very new players. Very new players have no culture items that can be helped (unless they bought the one for diamonds or started during an event) and don't usually benefit that much from extra culture (if a very new player has an event building, it probably provides plenty of culture without neighborly help. and only have two levels of builders hut. Building times are already extremely short, so motivating builders doesn't make that much of a difference. Basically you end up with a lot of coins, which are nice, but you probably get a lot of coins from residences anyway, and without a trader you have nothing to do with them. Even with a trader, coins are of limited utility early in the game. Usually you can't return most of the neighborly help because it comes from neighbors you haven't discovered, so you don't benefit from getting supplies.
Access to players willing to trade for your goods is actually far more beneficial early in the game than neighborly help, but Inno chooses to limit that through the use of the trader fee and limits on trade partners. I don't have a beef with this, just pointing out that Inno's primary motivation in their design of features in which players interact with each other does not necessarily seem to be to make gameplay more effective for new players.
Neighborly help is much more beneficial to players who have medium to large cities with many cultural buildings and/or have a higher need for coins because they are more advanced in the game.
In sum, retaining new players seems unlikely to be a motivation for Inno wanting to encourage neighborly help. If it were, surely they could have developed something more effective. So ... why do they want to? If they do?