Space monsters and pirates seem to always target me versus my opponents. What I've done is spawn the space monster near an opponent's core world. At least maybe the opponent will attack the monster and give me some time to move my fleets

Hey, that's not the case.

If you use cheats to disable fog of war, you will see ai suffering from pirates as well. AI also does not have omni vision, so in early game stages, their ships can be taken by surprise just as much as those from human players. Usually this feeling of AI being able to circumvent pirates/space monsters comes from the AI prioritizing colonization a lot. So that out of 10 ships they build 8-9 will make it to destination. Which would be about the same number for a player, when colonizing. 

Yeah if you ever see a large number of monsters it's usually due to the AI feeding ships to the monsters. I exploit this mechanic all the time. When spawning a monster you have to keep in mind the monsters go last on turn order. If you put the monster right next to the AI ship the AI will generally be able to get away. But if you lead the ship your trying to attack the monster will generally attack it. 

It is turn 22

16 players on the map.

And the pirates seem very selective on who they target. (I am Altarians)

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    Line 1799: Debug Message:  CBattleSystem::AddBattle ticket 13 attackerID 1062830 (Fleet 1062830) owner Pirates ID 1052752 defenderID 2113240 (Torus Fleet I) owner Torian Regime
    Line 1810: Debug Message:  CBattleSystem::AddBattle ticket 14 attackerID 1059452 (Fleet 1059452) owner Pirates ID 1052752 defenderID 15985 (Fleet 15985) owner Torian Regime
    Line 1822: Debug Message:  CBattleSystem::AddBattle ticket 15 attackerID 1059452 (Fleet 1059452) owner Pirates ID 1052752 defenderID 15985 (Fleet 15985) owner Torian Regime
    Line 1921: Debug Message:  CBattleSystem::AddBattle ticket 10 attackerID 1062830 (Fleet 1062830) owner Pirates ID 1052752 defenderID 15985 (Fleet 15985) owner Torian Regime
    Line 1932: Debug Message:  CBattleSystem::AddBattle ticket 11 attackerID 1059452 (Fleet 1059452) owner Pirates ID 1052752 defenderID 15985 (Fleet 15985) owner Torian Regime
    Line 1945: Debug Message:  CBattleSystem::AddBattle ticket 12 attackerID 1059452 (Fleet 1059452) owner Pirates ID 1052752 defenderID 15985 (Fleet 15985) owner Torian Regime
    Line 1956: Debug Message:  CBattleSystem::AddBattle ticket 13 attackerID 1062830 (Fleet 1062830) owner Pirates ID 1052752 defenderID 15985 (Fleet 15985) owner Torian Regime
    Line 2165: Debug Message:  CBattleSystem::AddBattle ticket 17 attackerID 1062830 (Fleet 1062830) owner Pirates ID 1052752 defenderID 15985 (Fleet 15985) owner Torian Regime
    Line 3378: Debug Message:  CBattleSystem::AddBattle ticket 22 attackerID 2109677 (Fleet 2109677) owner Pirates ID 1052752 defenderID 2107493 (Fleet 2107493) owner Corporate Sector
    Line 3466: Debug Message:  CBattleSystem::AddBattle ticket 30 attackerID 8404335 (Capro Core Fleet IV) owner Pirates ID 1052752 defenderID 15798 (Fleet 15798) owner Corporate Sector
    Line 3477: Debug Message:  CBattleSystem::AddBattle ticket 31 attackerID 8404338 (Almaak Fleet VII) owner Pirates ID 1052752 defenderID 10500565 (Fleet 10500565) owner Festron Hunt
    Line 3488: Debug Message:  CBattleSystem::AddBattle ticket 32 attackerID 2109677 (Fleet 2109677) owner Pirates ID 1052752 defenderID 9452146 (Fleet 9452146) owner Corporate Sector
    Line 3499: Debug Message:  CBattleSystem::AddBattle ticket 33 attackerID 1061121 (Fleet 1061121) owner Pirates ID 1052752 defenderID 7352996 (Fleet 7352996) owner Altarian Republic
    Line 3511: Debug Message:  CBattleSystem::AddBattle ticket 34 attackerID 8404335 (Capro Core Fleet IV) owner Pirates ID 1052752 defenderID 15798 (Fleet 15798) owner Corporate Sector
    Line 3726: Debug Message:  CBattleSystem::AddBattle ticket 39 attackerID 9445527 (Fermento Fleet IV) owner Pirates ID 1052752 defenderID 3161408 (Nike Fleet I) owner Festron Hunt
    Line 3738: Debug Message:  CBattleSystem::AddBattle ticket 40 attackerID 4204635 (Fleet 4204635) owner Pirates ID 1052752 defenderID 6304981 (Fleet 6304981) owner Terran Alliance
    Line 3749: Debug Message:  CBattleSystem::AddBattle ticket 41 attackerID 1061121 (Fleet 1061121) owner Pirates ID 1052752 defenderID 7352996 (Fleet 7352996) owner Altarian Republic
    Line 3876: Debug Message:  CBattleSystem::AddBattle ticket 37 attackerID 1063215 (Noldor Fleet III) owner Pirates ID 1052752 defenderID 16039 (Fleet 16039) owner Altarian Republic
    Line 3890: Debug Message:  CBattleSystem::AddBattle ticket 38 attackerID 8404338 (Almaak Fleet VII) owner Pirates ID 1052752 defenderID 13799 (Caridea) owner Festron Hunt
    Line 3899: Debug Message:  CBattleSystem::AddBattle ticket 39 attackerID 9446620 (Strife Fleet IV) owner Pirates ID 1052752 defenderID 1062817 (Fleet 1062817) owner Krynn Syndicate
    Line 3912: Debug Message:  CBattleSystem::AddBattle ticket 40 attackerID 2109677 (Fleet 2109677) owner Pirates ID 1052752 defenderID 5253550 (Fleet 5253550) owner Corporate Sector
    Line 3923: Debug Message:  CBattleSystem::AddBattle ticket 41 attackerID 1061121 (Fleet 1061121) owner Pirates ID 1052752 defenderID 7352996 (Fleet 7352996) owner Altarian Republic
    Line 3937: Debug Message:  CBattleSystem::AddBattle ticket 42 attackerID 1061122 (Fleet 1061122) owner Pirates ID 1052752 defenderID 6302693 (Fleet 6302693) owner Arcean Republic