There are of course two sound commercial reasons for increasing single fares by disproportionately higher amounts. Firstly, a passenger making a single journey is less likely to be able to switch to other forms of transport - in particular, if they only want to go one way, they can't really go by car. Secondly, in the case of a passenger making a return journey with various operators to choose from, or indeed a journey where the passenger doesn't know if they'll need to come back or not when they travel out, a low extra cost for a return encourages the passenger to purchase it, thus tying them to that operator for the return trip too.
But equally such fares are clearly contrary to the public interest and the long-term growth and integration of public transport systems.
Raising single fares, while discounting day and weekly fares, has the effect of inhibiting cross-operator trips; consequently reducing the scope for growing public transport as an overall offer.
Moreover, there is a strong public disbenefit in prospective riders not getting on the first bus that comes along towards their destination. This reduces bus occupancy - and hence distorts consumption of road-space and stop space (a high-cost public resource), and imposes unnecessary delay into journey times.
Ideally, public transport should allow seamless transition across operators and modes; with no cost penalty or advantage either way.
Effectively, fare setting in the deregulated bus market follows monopolistic principles - as almost all routes are monopolies, and operators try so far as possible to remove competition (by fair means of foul) wherever possible. Hence - in accordance with standard monopoly pricing theory - operators are motivated to restrict services and drive up fares; so as to prevent the overall market from growing to a point where a competitor might find market entry an economic possibility. Fares are set so as to grab as much as possible of the existing market; while avoiding pursuing any growth in the overall market.
Sometimes, on very particular high volume routes, an operator can buck the general principle - as with First's 'Quids In' flat rate single fare along Oxford Road. But that needs deep pockets, and is only done very selectively.