Programmers like to build their own ticket system because, having seen and used dozens of them, they know everything about it. That way they can stay in the comfort zone.
It's like checking out a new restaurant: it might be rewarding, but it carries a risk. Better to order pizza again.
There's also a great fact of decision making buried in there: there are always two reasons to do something: a good one and the right one. We make a decision ("Build our own"), then justify it ("we need full control"). Most people aren't even aware of their true motivation.
To change their minds, you have to attack the real reason, not the justification.
