Why, you ask! Three reasons:
WHAM! Although your supplement claims to have billions of live organisms, most of them are getting destroyed in your stomach. Stomach acid is seriously powerful—it can dissolve a penny. It has no trouble destroying delicate probiotics.
CRASH! Even if some of these probiotics get to your small intestine, typicaly they are just dumped into the first few inches. But your small intestine is over 15 feet long! There's no guarantee that ANY probiotics will make it all the way to your large intestine, where your worst problems may be lurking!
WHIMPER! So you're left with just a small band of brothers up against a large army of bad bacteria. To make matters worse, the probiotics in today's supplements are ofter NOT the strongest types. It's like sending the runt of the litter out to play tackle football against giants. Luckily, we've developed superior strains of probiotics that are tougher that steel when it comes to tackling bad bacteria.