Why is HMAC-SHA512 used on root the root seed to create master private key and chain code in HD wallets?
I'm willing to bet the 2048 rounds of hashing required by the BIP 39 Standard takes considerably more energy than 1 round of hashing. 

Put another way, substantial computing cycles are required to create brute forced Rainbow Tables when lower entropy human readable seeds are functionally mapped to private keys. This is somewhat analogous to the leading hash zeroes values miners satisfy to add blocks to a PoW-based blockchain, except the computing cylces are fixed by mnemonics-to-seed portion of the BIP39 Standard.

The Python stretch can effectively implement the mnemonics-to-seed functionality written into BIP 39.

stretch-&gt;passlib.pbkdf2(secret, “mnemonic”+salt, rounds=2048, keylength=64, sha512)

The two two Bitcoin-related examples of piped commands using stretch and bx yield the same result.

% stretch -f sha512 -r 2048 "bunker wreck real edge inflict aerobic buddy mercy divorce wolf bright immune fat foot poet section sustain reveal unique reflect have latin problem chapter" mnemonic123 | bx base64-decode | bx base16-encode | bx hd-new | bx hd-private
xprv9veSXAfA7GoYvaMAXgm3uYGS4VLH1Foc9qmTBRCb4c3F2Vqd261k2vo646nLU54MRVseFx2BbhZYQS9HDUy4dvTV3fygVGxedqYSR82B3he

% echo "bunker wreck real edge inflict aerobic buddy mercy divorce wolf bright immune fat foot poet section sustain reveal unique reflect have latin problem chapter" | bx mnemonic-to-seed -p 123 | bx hd-new -v 76066276 | bx hd-private
xprv9veSXAfA7GoYvaMAXgm3uYGS4VLH1Foc9qmTBRCb4c3F2Vqd261k2vo646nLU54MRVseFx2BbhZYQS9HDUy4dvTV3fygVGxedqYSR82B3he