Products like nexgard spectra are marketed for the convenience aspect. Owners struggle to remember dose intervals for different parasites and don’t like dosing multiple things. Because of this blanket use, most people are not aware of what is being treated and why or what the risk factors are for their pet or in their area.
canine and feline roundworm is a public health risk (it causes blindness in ~7 children per year and is increasingly being linked to learning difficulties). The eggs and infective larval stages that dogs and cats shed are not visible by eye. All pets should be wormed 3 monthly (monthly if direct contact with children ideally as this stops egg shedding all together).
fleas are annoying, can cause skin disease and anaemia in debilitated animals and are also very hard to get rid of once established in a household so most people want blanket cover for this.
ticks are hard to spot on an animal when they first attach, it is much better if they die or are removed within the first 24h, it is at the end of the feed (when they are big) that they are most likely to pass on infection such as lymes.
lungworm is relatively rare, but can be rapidly fatal in some cases.
you can’t blame people for choosing to use these blanket products for animals that live in their homes/sleep in their beds etc. I do think defra need to be looking more closely at pet parasitics with regard to the environment (but also bear in mind the widespread use of much stronger products on livestock which are non prescription and can be picked up from an agricultural merchant)