Question: howcome when I send a payment, most of my wallet becomes unspendable until the payment goes through?

Been experimenting with bitcoin lately. One thing that I find kind of weird and dont really understand is when I send a payment, it will send the amount specified along with the transaction fee, but then the majority (except for a very small fraction) of the rest of my wallet becomes unavailable until the payment is confirmed..

can anyone explain why this is?