[noggin]

ok, but with .srt you can change: size, position, color ecc ... i like more

Horses for courses.

Bitmapped subtitles, like Blu-ray PGS, allow for artwork that matches the movie, and in some cases multi-language Blu-ray uses this for graphics that aren't subtitles for dialogue but are instead graphics that are pertinent to the plot (like location captions etc.)

Rather than burning them into the video and then having to put a second graphic up as a subtile with a translation, they instead keep the video clean and use a forced subtitle to show a graphic tailored for the selected language. This graphic is positioned and designed into the movie - and using SRTs would make it look very odd.

SRTs are also bad at preserving position information. I've seen some PGS-style subtitles where the captions are positioned helpfully to avoid masking important picture content. SRTs don't do this in my experience.