Activation lock is automatically enabled when you turn on "find my" on your iPhone, but that's not the default - because Find My sends your location information to Apple which has privacy implications so no one would suggest it should be the default.
If you have Find My enabled you can brick your phone by reporting it as "lost". Without it I'm sure there is a good way to do this. Maybe they could allow marking it as "lost" without Find My, unless there's some technical reason I'm overlooking why that would be an issue. You definitely don't want to have the police be able to do this, or they would have the power to brick your phone anytime they wanted. Imagine being able to tamp down a protest against a government by bricking the phone of everyone in the radius of that protest!
It doesn't benefit manufacturers to have phones more easily stolen. The demand for iPhones is fixed - if I acquire a stolen iPhone then I won't need to buy one so it costs Apple a sale from me, the person who had their phone stolen has to buy another, so the number of iPhones Apple sells is the same. Yes maybe I would have bought a second hand phone but maybe that's what the victim does, in the end it doesn't increase their sales or their installed base.
Ironically what you are asking for (better blocking of stolen phones) WOULD be beneficial to manufacturers because then if an iPhone is stolen and is remotely bricked (like would happen if someone stole mine) then it is useful only for parts - and if re-use of parts from stolen phones is also blocked then it is worthless. But Apple would sell another phone because the victim still needs his replaced and the thief would end up tossing it in the dumpster when he realizes it has Activation Lock enabled.
Maybe Apple needs to encourage people to enable Find My more strongly or something.