There are many engines that use existing languages, with Lua being one of the most common scripting languages used by games. However usually when an engine has

There are many engines that use existing languages, with Lua being one of the most common scripting languages used by games. However usually when an engine has its own scripting language it is because the existing languages wouldn't fit and/or the custom language can work better with the underlying engine. For example if a game engine keeps track of asset references, scripts could be themselves assets like any textures, levels, materials, etc and have asset references as literals in the code that the script compiler understands, allowing automatic and precise extraction of asset dependencies the scripts (and the assets that refer to those scripts) can have. This is, e.g. what Blueprint does (BP is just a visual scripting language that uses a graph with a visual editor instead of parsing text to generate an AST but the same idea applies).