When The Spirit of Delta (the 767-200 bought for the company by the employees) was moved into one of DL's original hangars at ATL to become part of the Delta Museum,
the hangar was modified.
"Hangar Two is home to The Spirit of Delta, B-767. Because the tail of Ship 102 was four feet higher than the support beams in the “tail door” of the hangar, a team of structural engineers modified the structure in 2006 to allow Spirit to fit. The hangar also received a much-needed cosmetic change – a fresh coat of interior paint to bring the hangar back to its original “maintenance grey”. "
http://deltamuseum.org/M_Museum_Facilities.htm
How often do 757s need to be pulled into AA's hangars at LGA and other facilities where the tail does not fit? If it is not on a scheduled basis, then perhaps the cost isn't justified if AA can do scheduled work elsewhere. Presumably the equipment to lift the nose was purchased to accommodate the need - so unless that procedure doesn't work (sounds like it does), there is no need to modify the hangar.