Why not magnetic tapes in stead of vinyl records?

Yes, the time and cost of reproduction per unit assured the LP's market domination. And although unlimited money can be spent on a turntable to extract that last Nth of music and detail, the entry point for a reel-to-reel tape machine was always higher, and the mechanical complexity is much higher.

The reason for the 14" transcription LPs and 12" tonearms to play them came from a time when live radio programs and performances were recorded, mastered, and stamped to send out to radio stations all over the country. A radio station could easily stamp 100 or more transcription LPs and send them out overnight, where 100 tape duplications would have taken 4500 minutes, or 75 hours.

Also, there were until maybe 15 years ago, companies who produced high quality tapes copied at lower speeds. Barclay
Crocker was one of them. Their catalogue was all Classical music. Joe

Get both 2 is better than one.

Professional tape recorders and tapes exceed vinyl in resolution, signal to noise ratio and dynamic range. Do some research. And yes, tape duplication is more complicated than record stamping, but cassette recordings outsold vinyl disc immediately prior to the introduction of the CD format. So somehow and somewhere a few large companies figured out how mass produce magnetic tape recordings. Apparently it can be done.

Tape will never surpass vinyl in resolution and signal to noise ratio and dynamic range....

Nonsense.

Assuming the source is an analog session or master tape, tape to tape copying can produce a copy that's just one lossy step removed from the original.

OTOH, manufacturing a vinyl LP involves many more lossy steps:
1. the session/master tape is played through an equalizer circuit to impart the RIAA (or other) curve
2. the equalized signal drives a cutter head
3. the movements of the cutter head cut the grooves on a master disk
4. the master desk is used as a mold to produce a metal stamper
5. the stamper is used as a mold to produce a vinyl LP

Further, additional lossy steps are required for the consumer to play back the LP:
6. the stylus must track the modulations in the groove
7. the cantilever (which is never perfectly rigid, and which pivots imprecisely within an elastic suspension) must reproduce the movements of the stylus at the armature end of the cartridge
8. the cartridge converts physical motions of the cantilever to an electrical signal
9. the signal is reverse-RIAA equalized
10. the signal is amplified back up to line level.

Only now is the signal compable to the one coming from a playback tape deck, i.e., suitable for the line level input of a preamp.

Tape reproduction and playback can involve as few as 2 lossy transfers (record/play back). Vinyl reproduction requires at least 10.

Direct-to-disk LP recordings eliminate step #1. Even this small reduction results in audible increases in resolution, s/n ratio and dynamic range... which proves the point: every lossy step impairs realistic reproduction.

Whether any particular consumer tape setup is as good as a particular vinyl setup is a different question. Whether the cost of tape copies is affordable or the hassles worthwhile are different questions still. But tape is the inherently superior medium.