they steal everything.

Absolutely. Nazis are consistently creatively bankrupt with few exceptions.

• Original party aesthetics: rebranded from DAP to NSDAP (against Hitler's wishes) and adopted symbols to try and capitalize on the growing socialist movement after the failed German Revolution.
• Original party policies: A shallow syncretic mess. Even Mussolini thought they were morons.
• Neo-Nazi tactics: Also a syncretic mess, with tactics that contradict their own goals and abilities (e.g. copying black bloc, trying to adapt ex-military 1930s tactics to 2000s alienated teenagers).
• Aesthetics (both original and modern): Ignorant copypasting of ancient imagery (not-actually-Roman 'roman salute', neo-Roman architecture and statues, fake-Germanic runes)
• Nazi Pepe/Wojak edits: Both memes were already prominent from /r9k/ board.
• Fashwave: Vaporwave aesthetics copied without understanding what they represent and why they were interesting

why do we keep letting them steal?

Stealing is easy to do and takes effort to combat. There are things we've stopped them from co-opting, and plenty of contested symbols. But at the end of the day, when the mainstream media picks up a symbol and repeatedly assigns it to a group, it's not so easy to overcome that in broader society.