A single-ended amp has a single output tube that amplifies the whole waveform on its own, so there is nothing to split and no phase inverter in the circuit at all. Everything a push-pull amp spends on a splitter and a matched pair of output tubes, these circuits simply do without — one preamp triode, one output tube, one rectifier.
That is the recipe for the smallest amps: the tweed Champ and Princeton, built to be cheap student and practice amps of a few watts. Running the output tube in class A, a single-ended stage draws steady current and clips asymmetrically, so it favours even-order harmonics and starts to break up early and gently. There is no output-stage balancing to get wrong, which is part of why these amps became studio staples far out of proportion to their 5 W.
The cost is power and efficiency — a single small-bottle tube only makes so much clean volume — which is exactly why every larger amp in the archive splits phase and runs push-pull. Compare the splitters those amps use: the cathodyne and the long-tailed pair.