Uber's $40 billion valuation seems high, but the pre-IPO investment market has changed considerably from 2000, just before the tech bubble's burst, Carlyle Group co-founder and co-CEO David Rubenstein told CNBC on Tuesday.

"It does seem high, but remember, a lot of people thought Facebook's valuation before it went public was high, and it was much higher than that, actually," he said in a "Squawk Box" interview. "A lot of people have made money on companies like Facebook before they went public, and now people feel that investing in these companies before they go public is probably a pretty good bet."