From the restaurant's perspective I can kind of understand. They don't want to serve you a meal that they suspect you won't be able to eat because it's a bad look for their business. And if you complain, being too spicy is also way harder to rectify than being not spicy enough so they play it safe initially. Though it obviously sucks if you're specifically looking for spicy food.

This is why I advocate for chili oil, white pepper, and other condiments at the table or provided on request at Chinese restaurants. As a Chinese person who likes to fine tune the spice level of my food or change the spice level part way through. It also basically means the kitchen doesn't really have to worry about if something is too mild in the same way they don't have to worry if it's not salty enough.