The Abrams isn't as invulnerable as you'd think, if one got hit it in the roof, the belly or the rear, there'd be a good chance of it taking some serious damage.
The BMP's armour is enough to keep small-arms fire (anything up to about 14.5mm) out, as well as shell fragments, but it's main role is to deliver troops to the front line and give them fire support so it's not designed to go toe-to-toe with an MBT. The way the Russians would use them is en masse, along with MBTs. The tanks would draw much of the AT fire, allowing the BMPs to get closer to the enemy before the mounted troops got out, so they were designed to keep the troops safe from machine guns, snipers and artillery, the top infantry killers.