That’s interesting, but also a special case. Those were specifically underwater cables, they needed to be kept wet

And even after we created rubber, we definitely did not coat normal cables in rubber, ever since the properties of latex were discovered we’ve basically never had enough to fully meet the demand. Rubber is really useful for all sorts of things, and latex trees take a decade to start producing and only produce for another couple decades. Discovering alternatives using petrochemicals was a really big deal because of that

My point stands… The wires we used for early electricity and telephone transmission had minimal insulation, if any at all. Definitely nothing suitable to use underground until much later. Here’s a breakdown of the time frames for insulation (and note that cost was a significant part of the decision, even today we don’t use weather proof insulation unless the situation calls for it)

In the 1880’s the first insulated cables were insulated with gutta percha, a natural latex material produced from the sap of trees of the same name. This insulation needed to be kept constantly wet or it would dry out and fail to insulate the conductors. This material was largely replaced by rubber and vulcanised bitumen.

By the 1890’s mass impregnated paper insulation was being used on cables and voltages as high as 10kV.

In 1906 armoured cables were introduced which had flexible sheathing and two cloth covered, rubber insulated conductors.

In the 1930’s the first trials with PVC insulations were being made in Germany and by the end of the second world war there were significant varieties of synthetic rubbers and polyethylene.

By the 1950’s PVC was commercially viable and replaced rubber cables in many areas particularly in domestic wiring, aluminium was also starting to be used widely as an alternative conductor

The 1970’s heralded XLPE insulations replacing paper insulated cables in medium voltage applications.

In the 1980’s optical fibres were being introduced in overhead lines for data transmission and condition monitoring, and further use of XLPE in high voltage transmission lines between 66 and 240kV. Also high temperature super conductivity materials were discovered. PVC alternatives (LSZH cables) which were safer in fires were being developed in response to a number of tragic public fires which demonstrated the dangers of smoke and toxic gases from various materials including PVC’s.

By the 1990’s polymers were being extended to use in EHV, extra high voltage, applications and the use of optical fibres in overhead lines was becoming widespread.