Why are ticks so hard to remove?

Because they glue themselves in place.

George the tick
Ticks are remarkably well adapted for doing what they do best   latch on, hang on, suck blood. First they saw a tiny slit through the outer skin, using a pair of alternating saw blades called chelicerae, injecting an anesthetic as they go. The victim usually feels nothing. Next, the tick forces its multi-barbed hypostome deep into the skin and secretes a sophisticated organic glue thereby locking itself solidly into place. In terms of removal, the hypostome of a glued-in tick might just as well be part of your skin.