Usually, people say they are writing a new one because it's faster or easier or does something that the old one doesn't. Usually, they are also flat-out wrong about these assertions, and would have known it if they had spent a little more time looking at the existing solutions.

It doesn't harm me if someone who I don't work with writes their own templating system. But what if they put it on CPAN and add to the pile of half-baked unmaintained solutions that swamp anyone searching for a templating system there? What if I have to work on some of their code someday, and it uses this monstrosity?

And the more subtle issue is, how much better might TT be if people tried to improve it instead of writing their own half-assed version that gets tossed out in a year?

There are real reasons why re-inventing the wheel is usually wrong. Ask anyone who had to support someone else's bespoke templating system at their company. It's not just a knee-jerk reaction.