Well, let's look inside a little further.
A straight blowback design does just what the name implies. It employs Newton's law of physics that says "For every action, there is an equal and opposite reaction." When the trigger is pulled, the bullet flies down the barrel. At the same time, the slide travels backwards in the opposite direction. Keep in mind that the bullet might weigh, for example, 95 grains. Whereas, the slide weighs the better part of a pound. Add to that the force of the recoil spring and you have quite a bit of force to overcome. So, the 95 grain bullet may go flying at over 1000 feet per second, but the slide does not. It's simply harder to move the slide/recoil spring than that 95 grain bullet.
There is a limit to this design. The slide on my Makarov PM, pictured above, is very hard to cycle for most people. I have become used to it, but most women claim they can't cycle it. Even some full grown men comment on how hard it is to rack this slide. It's really not a big deal once you get used to it, but I understand the comments. The reason for all this force is the 9x18 cartridge. It is widely considered to be the highest pressure cartridge you can fit into a blowback design without making the slide huge or the recoil spring horribly stiff.
You may be familiar with the Hi Point pistols. These guns throw conventional wisdom out the door and just make the slide so big and heavy that even a .45 acp operates using the blowback system. 
Obviously, the guns become very large and bulky by the time you get to .45 caliber. Even the .380 pistols are pretty darn bulky. I have fired the 9mm and thought, while it operated ok, it was just really unwieldy.
The solution to this conundrum is the Locked breech. In a locked breech, the barrel "locks" to the slide when closed and only unlocks once the bullet has left the barrel and pressures have dropped to a safe level. This allows a pistol of reasonable size and weight to chamber a powerful round like the 9x19 or the .45 auto. In fact, some guns in .32 and .380 now use locked breech designs so that they can shrink to pocket size.
The cutaway below shows the classic 1911 design by John M. Browning. You can see the locking lugs of the barrel are in the corresponding grooves of the slide.
The next photo shows the toggle link that allows the barrel to drop and disconnect from the slide. This is designed to happen only after the bullet has left the barrel and pressures have dropped.
In the next drawing you can see that the barrel has dropped out of the way and unlocked from the slide. The slide has continued rearward and is now ready to move forward and load a new cartridge.
Without the locking system, you would have to make the recoil spring so stiff as to make the pistol unfeasible. Either that or increase the size and weight of the slide. Again, basically ruining the practicality of the gun.
There are plenty of variations on the theme, but you get the point.
