Why is it that in text book circuit analysis the “load” of circuit is always a resistor?

What if in my circuit my next stage is a speaker, or a lamp, or another Opamp circuit?

Answer:

In short it is to ease the circuit analysis.

The way the circuit was drawn implies that the analysis “assume” that whatever being connected to amp output eventually will not affect the behaviour of the circuit. Actually, the load (in this case, R1) can be anything that demand current within the capability of the amp, and if the load is not, then we just need to insert “something” or "buffer circuit" in between the amp and the load to ensure the behaviour of the circuit remains the same. This is what you would call divide and conquer – analyse/design one stage at one time.