What the tape sees and what the input of the receiver/preamp/integrated amp/AVR sees are very different. Line level was basically standardized because different brands made their models of equipment to work with their own products, but not necessarily with someone else's because it was a good business model and because it was a way of saying "That won't work" when a different brand of source or amplifier was used. Once people started making their own choices and tinkering with something in order to make it work with something or in a way that was never intended, the manufacturers decided that it was in their best interest to standardize. That's when the marketing departments started working their magic. Yay.
A tape head sends very low voltage to the tape because it can't handle much before distortion becomes a huge problem but once it's recorded with full frequency range and low distortion, anything else is academic WRT impedance mismatches, gain/output level and allowing enough headroom to be useful.
The original standards for just about all audio equipment started with low impedance/balanced input/output and was mostly commercial/industrial/reference equipment before seeing demand from consumers. Also, it would have been unusual for one piece to be connected to another, so universal specs for consumer goods weren't really needed. Once consumer goods were being designed, they found ways to make them at lower cost and changing to unbalanced/high impedance made sense because none of the equipment was far from anything else in the system and often, it was in the same cabinet.
