[nosianu]

How? A glider climbs when the air it is in rises faster than the airplane sinks in that air to maintain its forward speed.

If you pitch up you can only exchange speed for a little bit of altitude, briefly.

[jacquesm]

Exactly. A glider in air that doesn't rise is like a yo yo on a string, you need to add energy to the system (rising air, pulling up on the string at the right moment) to be able to overcome the eventual return to the ground state.