That comes with disadvantages in that reading the PKGBUILD is inherently unsafe, and it was the cause of many concerns back in the days with tools like yaourt, which pretty much just blindly sourced it to get the variables out, which means immediate code execution just loading it from the AUR.
However, the AUR helpers in question, which are not official tools, were to blame. Some developers of these tools could not or did not want to solve the problem. According to wiki.archlinux.org/title/AUR_helpers, almost no AUR helper sources the files automatically nowadays.