This appears to be a duplicate of Why does connecting flight need a security check at connection airport? but the accepted answer IMO simply doesn't apply to my specific example.
In many "good" airports arriving passenger simply exit the gate into the departure area and they can either walk to their connecting gate or through the baggage claim and out the door. For example in FRA if you arrive from the US in concourse Z and you transfer to a Schengen flight in A you just go through passport control between Z and A but not through security again. European airports like this are BRU, FRA, MUC, VIE, ZRH, etc.
Examples of the "bad" layouts in Europe are LHR or CDG. All connecting passengers get funneled through long corridors to a single collecting point where they have to go through security again before they can enter the departure area.
- It's bad for the passenger in terms of inconvenience and time. LHR seems to be particularly annoying: long lines that are poorly staffed, no priority lanes, etc.
- It's bad from a Covid mitigation perspective: being jammed in a line for a long time is probably the single riskiest activity of travelling. Social distancing in security lines is NOT practical.
- It's bad for the airlines, since they have to increase their minimum connection times and have to deal with way more variable transfer times and missed connections.
- It's bad for the airport since it takes up a lot of space, people and money.
So this begs the question why? The accepted answer states
There are locations that fly to LHR that do not conform to international norms regarding international safety.
This may be true, but it would apply to other airports the same way. They aren't a lot of location that fly to LHR but not to FRA. There ARE flights from Lagos to FRA as well.
To clarify the question: what is different between, say, LHR and FRA that would cause LHR to add significant cost, stress, time, insecurity and inconvenience to a connection? Why does LHR think this is necessary and FRA thinks it isn't?
I focused this questions specifically on European airports to make the comparison as "apples to apples" as possible. I've seen discrepancies in other airports as well. In DOH and IST I had to go though security recently but I don't think we had to in DXB. KEF is also a no. US & Canadian airports are different since they have no international transit. Domestic transfers do not require security.
Overall this seems mostly random with no obvious reason to do it one way or the other. Hence the question: is there a rhyme or reason ?