You've got to use something to get from zero to 25,400 feet/sec, and rockets work damned well -- especially if you're putting big stuff into orbit. The advantage of rockets is that you can stage them -- drop off the parts you don't need once you're done with them. To do the same with a "space plane" means you're going to launch 250,000 lb into orbit, and bring 200,000 lb of it back to Earth. (This is what the Shuttle does....)
2. Why does the shuttle come in at such a high rate of speed? This seems excessively risky to me, and I don't understand the reasoning for it.
Look at it this way: to slow the satellite down from 25,400 fps to a "reasonable" speed of 5280 fps would require about 170,000 lb of propellant -- roughly the weight of the Shuttle itself. It would be ruinously expensive to launch that much propellant into space merely for re-entry. So they use the atmospheric drag instead. The deorbit burn uses a relatively modest 9800 lb of propellant, to bring the Shuttle into the "thick" air.
1. Why do we use rockets to launch spacecraft?
r9: You've got to use something to get from zero to 25,400 feet/sec, and rockets work damned well -- especially if you're putting big stuff into orbit. The advantage of rockets is that you can stage them -- drop off the parts you don't need once you're done with them. To do the same with a "space plane" means you're going to launch 250,000 lb into orbit, and bring 200,000 lb of it back to Earth. (This is what the Shuttle does....)
Let's not overlook the fact that a substantial part of the ascent cannot use jet engines, because they breathe air to operate, and above a certain altitude, there ain't none.
Many proposals for a jet 'truck' to take the spacecraft high into (certainly not above!) the atmosphere and to a modest (~500kt) speed have been floated, and small satellites have in fact been launched this way. However, nobody knows (yet) how to make a practical jet truck big enough to take a craft the size of the shuttle high enough and fast enough to be worthwhile.
Isn't it also true that, long before its speed were reduced to something reasonable, the shuttle would begin dropping like a rock toward Earth? I'm referring to the loss of centrifugal force which had maintained the orbit.