Because immigration really really don’t want to do it. There have been some very few successful extensions but I’d say 99% of holders just bounce out and back.
Er? No it isn’t. There’s one side for remote work, but there’s the “soft power” side that allows for Thai cooking classes, or Muay Thai or medical or dental work.
Yes you can apply via Manila if you are currently in the Philippines. They will need proof that you are and you’ll need to stay there until the visa is approved.