"Contribute to obesity" and "bad for you" are not necessarily the same thing. Which one are you asking about?

Some sugar substitutes are bad for you for reasons unrelated to weight gain, such as messing up your gut microbiome.

There is some evidence, although it's not conclusive, that sugar substitutes may be linked to weight gain. The idea is that your body associated the sweet taste with an increase in blood sugar and thus expects an increase in blood sugar. When you drink a sugar free drink, you get the taste but don't get the blood sugar increase, so your body makes you crave sugar to get that increase it was expecting, and you end up consuming more calories than you otherwise would have.