There are lots of reasons.
Some students refuse to use any differentiation, even if they need it or it benefits them. They don't like to be different to their peers.
Contrasting needs. I teach 11 different classes per week of 34 students each. An average class will have about 7 children with SEN. My bottom sets might have upwards of 25. This year I will have a class with a dyslexic child who needs everything with a blue background, and a colourblind student who cannot see blue and will need a white background. I usually have 1 student with ASD who needs a calm environment, but I'll have the parent of the ADHD student complaining that my lessons need to include more games, to then have the ASD student have a meltdown, so SLT tell me I need to have no games in my lessons as student with ASD needs quiet. Meanwhile (and this is true) I have the student with SEMH complaining to SENCo that the ticks from student with ASD is making them anxious. So how can you win when dealing with this?
Next, and I hate myself for saying this, but time and resources. We use a black pen because they are the cheapest and budgets are stretched. Pdfs go online rather than copying it up because I don't have the time to copy it up. I can't email every parent of every dyslexic child their homework, because I don't have that kind of time. I will upload it to the vle though and expect the parent to keep on top of that system if they need to prompt. Planning takes a massive chunk of time, but when you have the sheer number of needs that we are dealing with, I could literally be spending all my time planning and creating differentiated materials. But in the 10 hours per week I have after school I need to: run detentions, mark 3 classes of books per week to keep on top of the marking policy, attend meetings, parent's evenings, open days, plan lessons, call back parents, keep on top of my displays, create and update schemes of learning, run revision classes, intervention sessions with students who had covid/were sick so missed loads of learning and now can't keep up, mark assessments, analyse results, transfer results to central system, etc. If we ignore the outlier classes with 25+ SEN and say an average of 7 students with SEN across 11 classes, that's 77 adjustments I need to put into place into every single activity I do, plus homework. That's not sustainable, and as PP has said, all dyslexic students are not the same and their needs can vary massively. So we try and do the little things like upload the slides and homework to vle, I make the font comic sans, I have written as well as verbal instructions but it usually isn't enough.
Rather than saying "why aren't teachers doing more?" You need to be saying "why aren't governments funding more?". But parents know it's easier to get teachers to work longer and longer than it is to get fight for the funding schools desperately need to make classes smaller, to provide professional training by experts rather than Mr Harris in geography who once got a grade 8 out a dyslexic student some years ago, to hire more support assistants, to buy software and devices such as tablets, laptops and reading pens to support learning, and to retain more experienced staff so new teachers can learn from them. Even things like money for overlays, coloured paper and printing needs to be considered.
This isn't just a problem impacting dyslexic students, it impacts everyone in the room.