Sed exists everywhere for a reasonably complete definition of everywhere - if it doesn't exist then you probably don't have access to Bash (and definitely not a fully featured one)
I have managed networks with 1000+ (microwave link) network devices that had Bash but no sed.. Sure was nice to be able to script maintenance on those..
Your definition of "reasonably complete" might not work for everyone else's use case.
Also readability: While some of the examples may be somewhat confusing to someone who doesn't have a lot of Bash experience, to someone who does it can be a lot more readable then feeding a string in yet another language to an external program and processing the output..