In BT's case it is probably to minimise the number of complaints about not getting over 40Mbps on the 80/20 product.
Remember, it is only in the last few months that 40/10 went self-install. As the effect of poor home wiring or a poor line will be more marked on FTTC than it is on ADSLx there could be a lot 80/20 lines that perform well below their best with a self-install, where an engineer install eliminates home wiring issues and should also test the resultant sync. But - isn't Infinity 2 also self-install now?
Sky may be similar. For a long time they didn't even have 80/20 on their website. Maybe they want to keep peak time throughput down if they can.
Plusnet still have engineer install on 80/20 and 40/20. (Note, 40/20 not 40/10). They only take 80/20 off Openreach, and cap the download for the 40/20.
My broadband basic info/help site - www.robertos.me.uk
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Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 56.6/14.1Mbps @ 600m. - BQM
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