One problem is the simplistic calculators' weaknesses noted above. Industrial strength financial planning programs without these weaknesses that also do IRA comparisons are available. And they require many more inputs than the typical IRA calculator. These are the reasons their results are realistic and why you aren't likely to see them on-line. Another problem is inadequate warnings about the potential for non-real world results because of the calculators' weaknesses.
Several calculators are at mutual fund web sites.
A typical disclaimer only notes that past performance is no guarantee of future performance. Maybe they dug up a mutual fund prospectus disclaimer and used it for the IRA calculator. A thorough explanation of the calculators' weaknesses and assumptions is needed for the average user and may be prudent for the providers. The calculators' faults appear to be recognized by the big name financial organizations offering web site and low cost calculators. The San Jose Mercury News published an excellent article written by Mark Schwanhausser on IRA calculator weaknesses.