There is another one. Frustration. Ive Seen many Guys, Most of them zerg, who Just dont have the Patience or nerves to Play that Game. Sometimes they think they can bust through and to lose their Army and getting counter Attacked. Some Just dont want to Play against that at all since its so boring and you need to do so much more than the Turtle. (This reads a Bit Like His Number 1 but i wanted to get to the psychological aspect).
And the other one i have mentioned too. You can Turtle into going for a Kind of Tech as a win condition. Hoping to win or doing enough damage so you have the Advantage in a Low Eco Game.
but yeah mostly its about massing a huge Army. But if someone reads: plays defensely with defending mechanics rather than building an Army and attacking the opponent into: never does anything Else except sitting Back and defending, then ill ask Them where the win condition is because in their source there is Not Stated a win condition. It Just vaguely explains the Playstyle.
Why do you harass as a Turtle ? Because your opponent cant Attack into you. You are Safe. You build the harass Units anyway. Hellions and liberators can be used and are used in lategame mech armies. So you could Just harass with Them right away. Your opponent will quickly as possible expand and tries to swarm you. You can prolong the Moment where He has the means to do so. Especially zerg is Sometimes so easy to harass. Just Shift Set Up a bunch of liberators and while the zerg tries to kill the libs with Queens you Hit Them with a hellion Squad.
If i would use His Definition strictly then there are Like No turtles above 3k mmr because they all will do at least Something Else to harass. A Banshee. A liberator. Hellion runby, hell, even a BC. But it doesnt Change the fact that their Playstyle is Camping with a Lot of Sensor towers, turrets, siege Tanks and planetaries so you have No Hope in attacking into Them early but also that there is Not threat coming from them early.