D&D 5e is premised on the idea of bounded accuracy.
... bounded accuracy means your attack bonus/defenses don't automatically increase with level ... bounded accuracy lets you take a monster (say, an orc) and pit him against pretty much any level of hero, knowing that the monster will still have a decent chance to hit and dodge.
magic weapons and armor typically won't give more than a +1, with +3 being the cap and representing things of artifact power.
Your DM is playing the game as it was intended.
The difference between a 1st and 20th level character is not primarily in their AC, ability to hit or saving throws but in their hp, damage output, available options and methods of "pushing" the action economy.
As for magic items, +1 armour is a rare item under the DMG. On p.38 it suggests that 9th level characters in a low and normal magic campaign should have no magic items at all and in a high magic campaign they might have one uncommon magic item - they might get a rare item by level 11 in a high magic campaign or level 17 in a normal magic campaign.
Based on the same table, each player should have starting equipment plus 525-750gp - not enough to afford non-magical plate armor anyway. They should be able to afford that by level 11.