Because primary sources are highly valued in philosophy, and unlike math or science, those sources never become obsolete.

Also, contemporary philosophers trying to restate the ancients often distort the originals, or do so in an inferior way.

Philosophy is not a mathematical or experimental result you could just summarize or restate in an equivalent way without distorting the meaning.

That's not to say that summaries or restatements don't have their place in philosophy too, but to really get what these philosophers were saying there is no substitute for reading the originals.

One other thing I want to add is that, unlike most other academic fields, philosophy is very much a conversation where the participants of today are still in dialogue with people who wrote hundreds and thousands of years ago.

If you were to limit yourself to just what your contemporaries said and wrote, then even if these contemporaries were honestly doing their best to summarize and restate what came before them (instead of distorting them and putting words in their mouth, which philosophers have a bad habit of doing) then you wouldn't be a fully informed participant in the conversation.

It's also a bit like a court of law, where you don't want to admit hearsay evidence, but want to hear directly from the people who were there.