Then I think that you're not giving Sustrans routes a fair go. Sure, some are rubbish but very often they follow delightful country lanes or (near me) straight through the Longleat estate, a splendid route which afaik didn't exist before Sustrans negotiated access. The newish tunnels near Bath wouldn't have happened without Sustrans. I cannot see many councils creating bike routes by themselves, can you?
You can't compare it with France, etc but then many EU countries have a long tradition of cycling whereas here it's only just become popular.
The big issue, for me anyway, is that you've got no idea what you're getting until you get there - I've posted this before, but NCN 66 near me is a prime example of a good idea poorly executed.
Leave Leeds heading (roughly) east and you're on either on road cycle lanes or shared space.
Then there is a bit of off road, before getting back onto tarmac - it's only a few hundred yards and is doable on a road bike but only just at this time of year.
More shared space, then it just ends in a dead end at a kerb. Up that and across a bit of footpath and there are some vague signs directing you on a quiet but badly potholed road for a bit.
You then cross the dual carriageway A6120 at a toucan crossing and hopefully spot the signs directing you on road up some residential streets.
It's then a bridleway onto an office park, across there and back onto the bridleway which is mainly hard packed and well drained, but has some very muddy patches, through some woods on a similar surface before eventually dropping down onto a tarmac road. It's then tarmac roads with no specific cycling provision for the next mile and a bit.
Then you're onto a rough, rutted and rocky bridleway up into some woods, where it gives way to mud. At this time of the year, that's several inches deep mud with lots of standing water.
Then, after another mile it drops down onto a tarmac road and stays on tarmac for the next few miles, initially on road and then with the option of on road or shared space path. Then it dumps you onto another rocky, rutted bridleway for a bit, before more tarmac, before ducking into another wood on another bridleway with it's own fair share of muddy lumpiness before you get to Wetherby
The point I'm making is turn out for that on a road bike, or a trike, and sections of it will be difficult and other parts will be simply unrideable.
Turn up on a mountain bike and while you're ideally equipped for the muddy or rocky bits, you've got an awful lot of slogging on smooth tarmac to deal with between the fun bits.
Whatever bike you turn up on, it's the wrong one for significant sections of it and Sustrans don't seem interested in making the necessary information available to anyone.