While PNG is definitely not as simple as TGA, I'd say it's "simple" in that it's spec is mostly unambiguous and implementing it is straight forward. For its relative simplicity it's very capable and works in a variety of situations.
One nice aspect of PNG is it gives a reader a bunch of data to validate the file before it even starts decoding image data. For instance a decoder can check for the magic bytes, the IHDR, and then the IEND chunk and reasonably guess the file is trying to be a PNG. The chunks also give you some metadata about the chunk to validate those before you even start decoding. There's a lot of chances to bail early on a corrupt file and avoid decode errors or exploits.
A format like TGA with a simplistic header and a blob of bytes is hard to try validating before you start decoding. A file extension or a MIME header don't tell you what the bytes actually are, only what some external system thinks they are.
One nice aspect of PNG is it gives a reader a bunch of data to validate the file before it even starts decoding image data. For instance a decoder can check for the magic bytes, the IHDR, and then the IEND chunk and reasonably guess the file is trying to be a PNG. The chunks also give you some metadata about the chunk to validate those before you even start decoding. There's a lot of chances to bail early on a corrupt file and avoid decode errors or exploits.
A format like TGA with a simplistic header and a blob of bytes is hard to try validating before you start decoding. A file extension or a MIME header don't tell you what the bytes actually are, only what some external system thinks they are.