Because the Rafah crossing is currently controlled by the Israeli army. Egypt can allow all the aid it wants, it’ll still get stopped by the Israelis.
> But recent intelligence has shown the extent to which Hamas has been able to build many of its rockets and anti-tank weaponry out of the thousands of munitions that failed to detonate when Israel lobbed them into Gaza, according to weapons experts and Israeli and Western intelligence officials. Hamas is also arming its fighters with weapons stolen from Israeli military bases.
> “Unexploded ordnance is a main source of explosives for Hamas,” said Michael Cardash, the former deputy head of the Israeli National Police Bomb Disposal Division and an Israeli police consultant. “They are cutting open bombs from Israel, artillery bombs from Israel, and a lot of them are being used, of course, and repurposed for their explosives and rockets.”
https://www.nytimes.com/2024/01/28/world/middleeast/israel-h...
And quite a bit of what's being talked about wrt to re-manufacturing Israeli unexploded ordance was stockpiled prior to Oct 7th. Israel did not start bombing Gaza only after Oct 7th (and in fact had been bombing Gaza as late as Sept 23, 2023).
Gaza is on the ocean: they could have been brought in via boat.
Otherwise the answer is largely tunnels.
Before May 2024, Egypt was the primary route for aid to get it, but getting from Southern Gaza to northern Gaza was incredibly dangerous.