My first plants did the same thing and I've learned the hard way over the years. A few things may factor in here. Too much fertilizer and not enough pruning. As you've noticed, they have a tendency to grow straight up and keep going.

Fertilizing is fine for seedlings if kept pruned. I've always lopped mine off to encourage branching and most importantly, a very sturdy plant. You can keep a single leader branch but I would cut the entire plant back to no more than one foot...maybe shorter. They will come back fast and strong and you will have a more attractive plant to boot. Starfruit, when/if they begin fruiting, can often overproduce and overwhelm the plants branches... resulting in broken branches or even loss of the tree.

Keeping the central leader will give you a good platform to later do a possible graft attempt with an established cultivar.

High nitrogen fertilizer can make them grow tall and skinny. Also not enought sun/light will also cause that to happen. Some of my If they get enough light they should start getting a thicker stem.