Martinez is on something like 1m a year in salary so naturally he will be a lot easier to sell than Rojo on 5m net, or whatever he's on. Rojo and Romero have suffered from injuries and poor fitness for a long time and Lingard has legitimately been one of the worst players in the division for some time now.
Martinez run of form since Leno was injured is above and beyond anything any of those players ever showed for us, with perhaps the exception of Lingard's one great 6 month spell that one season...
But Chelsea, Arsenal, City and Tottenham also have serious issue when trying to sell players, especially now that corona has bankrupted half of Europe's clubs.
Chelsea have about 10-12 actual dead weight players (Drinkwater, Bakayoko, Zappacosta, Arrizabalaga, Moses, Musonda, McEachran, Baker, Baba Rahman etc etc) all on insane wages, making them impossible to sell. Arsenal have like 10 central defenders and can't even give away players like Mustafi, Sokratis, Özil, Kolasinac, Guendouzi. Tottenham are desperately trying to offload Rose, Aurier, Carter Vickers, Foyth but actually managed to sell (albeit at a large loss) their deadwood in recent season because they pay their players peanuts. However, that's resulted in their squad suffering quality wise.
It's the drawback that comes with the incredibly high wages PL clubs can offer. It's hard to offload past it, unwanted players.
But let's not forget that we actually got 70m or so for Lukaku just last year, Blind two years ago for 15m and Johnstone - far inferior to Martinez - for 6 (!!) mil.