Investor enthusiasm for artificial intelligence has helped drive stock-market gains. - Getty Images Investors keep pushing the S&P 500 index to fresh all-time highs that leave it looking both expensive and top-heavy, with just 22 stocks in the index comprising half of its weighting, according to DataTrek Research. While the S&P 500’s lofty valuation has a lot to do with Big Tech stocks, its top 22 stocks by highest weighting extend to industries beyond technology, said Nicholas Colas, co-founder of DataT ...