The "US arsenal" was part of the 2 trillion investment that stood-up, equipped and trained the Afghan military that had everything to fight a war to defend their country except leadership and backbone. Money can't buy that. The inventory of equipment was surrendered by the "Afghan Military". Possessing equipment and knowing how to use it and more importantly the resources to maintain it are two entirely different things.
Biden was left with a settlement negotiated between the Taliban and Trump only, the existing Afghan government was not included in those negotiations. Exactly the involvement of NATO leaders, if at all, is not clear. Maybe Trump got a branding license for a new golf course somewhere out there in return ... the shit show that has resulted is what it is and for these reasons: a) the "negotiated settlement" between Trump and Taliban lacked any rational logistics, phased implementation and coordination with the national government; b) State Dept apparatus had been handcuffed to deliberately limit the number of Afghan helpers extracted into the US; c) the capitulation of the afghan military; d) the flight to safety of the Afghan president who wanted nothing to do with leading his country. Per Trump, the US was going to be out of there back in April/May. Biden deferred until August to try to resolve the dire situation he had been handed including an obstructionist residual Trump State Dept apparatus for vetting applications, assigning visas and evacuating Afghani helpers, US and other citizens and then the military.
Everybody is acting like this happened all of a sudden. It has not. It was public information to every NATO leader, contractor, US and other nationals on the ground. Every one of those national leaders bears their own responsibility for being caught offside in what was always going to be a shit show ... the only thing open to debate was how sloppy and sticky was this particular flavor of shit going to be.
Why US nationals were not the fuck out of there long before the final pull out of US military is insane and on them; every person still there bears a responsibility for their own well being. This phenomenon of ASS (arrogant, self-centered, stupidity) appears time and again in every high risk situation - whether it be a military / war scenario, a class 5 hurricane (those that decide to "tough" it out only to be left stranded on the roof of what remains of their home surrounded by sewage laden water with no food or shelter and then require others to assume risk to save them); a pandemic and anti-vaxers who decline to be vaccinated, then catch it, then expect exhausted and over burdened health systems to save their lives. People have the right to self-determine ... but they don't have a right to expect to be saved as result of their own ASS, this includes sucking it up when their self-determination contrary to basic common sense blows up in their face.
As a result of all this, US military and other militaries are now subject to grave risk of being bombarded with ordinates while maintaining the security of the airport. US tech is now running hyper surveillance over a broad area and is able to identify ISIS and other terrorist groups seeking to exploit. Sadly exterminating the vermin that would exploit this situation is resulting in collateral deaths and casualties. Once the US military finally departs, the Taliban are going get reamed out by the ISIS infiltration that is already occurring. Taliban v ISIS ... now that's a Match of the Day for you ... one set of cut-throats versus another. More proof that what goes around, comes around.