1) Young, fresh graduates who still remember well the CS material. Or those who have a lot of free time and can study for these interviews (for example look at this Quora link http://www.quora.com/How-much-time-did-you-spend-preparing-f... the first reply mentions spending 28.5 hours per week for 5 weeks, which is 142.5 hours total) From a senior software engineers perspective that's a ridiculous commitment for something where an arrogant asshole interviewer can shutdown a whole onsite interview process because they're having a bad day or have some favorite question they think is the One True Question for determining if someone is a software engineer.
2) The software industry tends to overvalue youth. It has a very high salary at start, but also a very quick salary cap. And most companies have no technical ladder, and even those that do it's a joke compared to the manager ladder. All of this is not surprising. Since the industry is so heavily biased towards hiring and rewarding youth, older experienced engineers are not valued.
3) Senior software engineers have heavy pressure to leave the software field. I'm feeling this now. I'm a senior at my current place and my salary is basically capped. There would be little to no benefit going to another company. And even if I wanted to, I have a family now and don't have the free time to study for software interviews. My options are basically go into management, consulting, or start a business. I have no desire to start a business or go into consulting because I simply don't like doing those business related things. I want to be an engineer not a business person. But if I just stay an engineer, I will probably get laid off someday due to ageism. And if I'm an old engineer who is laid off, job mobility is basically gone due to ageism and the CS heavy interviews that I don't have the time or motivation to study for. So the only safe path is management. A lot of senior engineers face this decision. Which again reduces the number of mentors and experienced engineers to teach the new younger engineers.
4) And doesn't anyone notice just how wrong whole concept of studying for an interview is? There's a whole book publishing industry dedicated to technical interviews such as Cracking the Coding Interview. The technical interview has become like the SAT. Many could just study to pass it, but be horrible engineers.