Why do IPO fetch such high share prices then tank?

I'm not so sure if it will definitely tank, but price volatility might be something that accompanies it. In fact, other than tank, share prices can actually shoot up because investors who did not get involved in the IPO because of some oversubscribtion. Over here, demand would exceed supply and your share prices will go up.

If your are referring to the "tank" period, maybe this could be after the lock-up period where the investors pre-IPO can finally sell off their shares. This could include senior management, employees, private equity firms and venture capitalists who might be eager to sell-off their shares and cash out. This is especially through for the PEs and VCs because that is their big return they are looking for. (This lock up period exsists because the firm does not want investors to have the impression that the original investors believe that the firm is over-valued)

An overestimation or overvaluation of the stocks also can cause the stocks to be "underbought", what happens here is that the underwriting banks might come in and buy up the shares to save face.

Companies typically want their share prices to be higher, before letting them them slide down, so at least investors won't get a first "bad" impression. So this might be another reason why share prices might fall.

Sometimes firm share prices after IPOs tank because people treat IPOs as speculative bets, and usually are not in it for the long term growth prospects of the firm. That's why after the "lock down" period expires, where these IPO investors are not allowed to trade their shares, these speculative investors will take profit and exit their investments immediately since they are just in it for the Short term momentum and inflated demand because of the IPO craze nowadays.

It is almost inevitable that there will be some selling pressures right after the lock down, but if you are in it for the long run, and the firm's business model is solid, you will see the prices surge back up and in fact surpass initial prices with continued strong growth.