People continue to confuse sane budgeting and running of a company with hubris.
This is a small studio by modern standards, considering the amount of platforms they are on. And mostly licensed properties. Ever work with licensed properties? The licensee isn't going to give you a budget for an expensive premade engine, and you become dependent on the engine maker to be available on your target platforms.
Why yes, let's re-write our entire engine just for HBO, who probably won't even renew the license in ~5 years time, on an HBO budget, who's working with us externally, lowering the budget even more. The cost of switching an engine could kill the company off - doing something like that is NOT free, or easy. Especially when you have to split your profits with another company while doing it.
I've barely played any of these Telltale releases but people truly do not know how games are developed, why it's not always advantageous to kill your in house engine. Bugs are a QA issue, and not something fixed by licensing another engine that also has bugs (and splitting your income even more)
Considering the good critical reception of games like Wolf Among Us, and the fact that they have multiple projects going at the same time, they're doing pretty good for their size.