Some of them actually do highlight specific changes, but Lego Group isn't consistent about it, even in sets of similar scale released around the same time.
For example, the instructions in the Lego UCS A-Wing from 2020 don't highlight changes; the Space Shuttle Discovery (released 2021) instructions do (with a red outline around the pieces added). The Shuttle set's a bit bigger, but the two sets are fairly similar in terms of scale and build complexity; it seems that it's really up to the designer to decide how explicit the instructions are (whether or not things are highlighted, how many things happen in each step, when specific parts are called out with arrows, etc.).
Tangentially, I own a LEGO fanmade book with a detailed guide on how to create instructions for your own creations. I don't have it on hand, but I recall POV-Ray was involved.
For example, the instructions in the Lego UCS A-Wing from 2020 don't highlight changes; the Space Shuttle Discovery (released 2021) instructions do (with a red outline around the pieces added). The Shuttle set's a bit bigger, but the two sets are fairly similar in terms of scale and build complexity; it seems that it's really up to the designer to decide how explicit the instructions are (whether or not things are highlighted, how many things happen in each step, when specific parts are called out with arrows, etc.).