May be for two reasons -
1. The first one is obvious, people of every generation would like to praise their generation. This has a first love impact - someone 60 years old probably would boast players from that era he has followed the game more passionately.
2. The counter effect of bullying. Among all major sports, cricket is played seriously by least number of countries, even lesser when it comes to history & sports literature. And, cricket is the most biased sports when it comes to glorifying the past, because it had been a monopoly of couple of countries (basically one intellectual theme), and the priority of the game has gradually gone down in those countries, hence the literature of cricket is extremely past biased. This has resulted a sort of comparison between generations, unfortunately in an ugly way.
I read lot on sports & their history - at least 5/6 major sports that I follow. That past vs present debate is everywhere; but I don't see cricket like bullying of past. In Soccer, Tennis, Basketball, Golf, Snooker, Baseball, Chess, F1 or Badminton (I leave field hockey here, because there is a major factor of playing surface), I read lots of comparison - Pele/Fat Ronaldo, Cruyff/Playboy Ronaldo; Federer/Laver, Borg/Nadal; Chemnarlein/Shak, Jordan/James; Nicklus/Woods; Ruth/Mayes/Bonds .... so on. But, I have never read such dismissal of modern players by the old greats like cricket - it's almost like every County team of 1900s had an Imran, Marshall, Viv, Warne, Akram..... in playing XI.
Besides, Cricket is most stat dominant game, therefore comparison is obvious & most common people are not capable of comparing stats with context. There are lot more technicality than just by stats to judge this game. One silly such example I can give is one of my recent debates on WK - I do agree Godfrey Evans is a colossus of the game, because he was beyond anyone among his peers, analyzing his stats, I did find something that tells me he was a brilliant gatherer of the ball, but not a mobile WK. GE has 173 catches in 91 Tests and around 165 innings - that's about 1 par innings & much, much lower than KAkmal, despite all the later's drops. But, people of his time'll tell that GE hardly dropped anything & they are actually not wrong. Now, GE played 54 of his 91 Tests in ENG, 16 in AUS & it can't be that in his era sincks to WK were lower - and, still he averages almost half to Kamran Akmal. I tried to find the answer for me first & found that Evans was so good in gloving that he could gather almost everything coming straight to him, he was a bulky guy & the peak speed of his pacers were not that express (I won't go to KM here for unnecessary distractions) for those reasons, throughout his career he kept almost always up on wicket - therefore, it was expected that any slight thick deflection, he isn't expected to catch & hence it's not listed as drops; rather his Captains kept a fine leg slip & first slip at what will be conventional WKs spot these days. Result, he had 46 stumping as well, which was for long time a WR. Now, I judge WK on overall package - his gathering, agility, keeping against spin, leg side saves, % of regulation catches, diving catches covering 1st slip & vacant leg slip, keeping against reverse swing Yorkers & big turner on rough ... in that regard 3 best ever WK in my order was Knott, Helly & Latif and indeed Evans is among best as well. But, I am believe in general one set will discard him for his 1 catch/innings, other set will glorify him for his keeping against Bedsar, Statham, Truman as well as Laker, Lock, Peebles standing on wicket.
Personally, I never, never disrespected any greats of any era. Just like Imran of 1980s, George Giffen was a great all-rounder of 1890s & KS Ranji of 1890s what's Steve Smith today or Viv Richards & Vic Trumper are similar batsmen beyond stats, parted by 75 years. In my every all time teams, I pick players on how dominant they were among their peers. I have a little bias for the eras I have watched, and I do use my judgement of overall quality of cricket by decades, but disrespect - NEVER.