Why so much?
I have 10% of my portfolio in Vanguard's REIT index fund, and VTSAX is another 3% to 4%, so I've got about 13% to 14% exposure, which I think is rather high. I believe REITs lost 3/4 of their value when the S&P lost half back in 2008-9.
A long answer, but hopefully it helps answer 'why so heavy' in REIT's...
I've been following the Case Schiller (sic?) reports for over a year, and the trend is up due to lack of housing inventory, and other factors that seem to form a long-term-trend - much like the long-term-trend of housing prices expanding in the San Francisco bay area. I foresee this long-term increase in housing prices becoming a trend in many other urban areas - Miami, Seattle, Denver, Houston, Dallas, etc.
Also, I'm worried that the markets will take a hit after the November election - regardless of the winner in the presidential race. Given I think the overall market will take a hit, I'm predicting the housing market's hit will not be as bad as other sectors of the economy. In fact, after the shakeout from 2007-2009, the housing sector looks to be one of the strongest sectors.
And, I have many friends, parents friends, etc who have made money over the years owning real-estate. I myself have made money on 7 of 9 homes I've owned, and I haven't even gotten into renters, lease agreements, etc. I would LOVE to have a 'fleet of rental houses' to provide long-term recurring passive income, but do not want to deal with the paperwork or the 'rental maintenance' hassles (even if I could hire someone to do-the-actual-work). REIT's are the best way I see of owning a 'fleet-of-rental-houses' to generate regular passive income without actually owning more houses, and hoping the houses I bought were purchases at the best price, at the right time, in the right location/location/location.
After last year's poor return on the S&P500, I'm hoping to beat that return long-term thru investing in REIT's.
Least important reason, but probably the highest emotional reason (and the reason I've spent so much time analyzing the markets): I'm close to my FIRE date, and feeling the need to do *something* that's not too risky to super-charge my savings/earnings. I see dividend generating REIT funds as a reasonable way to do that. Any large changes in the REIT market should be visible in the broader market indexes before real-estate takes the hit. [put another way - I've noticed real estate tends to be slower to react to downward pressure than the overall market].