I’ve never understood why. I remember seeing things from Squad saying that it was to help people “catch up” if they keep failing near the centre, but I think it just dilutes it. The game should have had a clear and easy progression that matches real life: Start by making something go upwards. Then ask them to make it go upwards and sideways. Then make something big enough that it goes upwards and sideways so much than it never comes back down.
THEN you can start letting them use science equipment etc.
I used to like the BTSM mod as it had a fantastic sense of progression, and worked in the way I dsecribed above. At the very start you have like 3 parts, and you need to use them to unlock the next one by going upwards, and then upwards and sideways etc. The first 12 tech-levels are a very cleverly balanced puzzle that is elegant and rewarding.
Unlike the basegame where a player can be practising how to control a Kerbal and accidentally pick up 3 tech levels and suddenly have 5x as many parts.
I was thinking of playing the a BTSM-spirtiual-like-successor Probes Before Crew, but I’ve not played it so I can’t vouch for how well the unlock progression is implemented.