Generally speaking, buying movespeed isn't 'making up for a lack of mobility'. Comparing movespeed increases to moveblocks is like comparing a bicycle to a car, and it's definitely not comparable to the potential for purchasing burst as a low-burst champ.
For starters, movespeed is painfully easy to cancel out, especially with the overabundance of ranged slows nowadays... it only takes a measely 40% slow to more-than-cancel out Glory's haste, only 30% to outdo Talisman. Moveblocks ignore slows, completely. Actually, they pretty much ignore harder (non-displacement) cc too, unlike movespeed boosts.
Second, movement via movespeed is WAY slower, and generally doesn't last long enough to make up for it. Glory will let a champ cover roughly ~475 units of extra distance over the 3 seconds (ex: ~375 MS x 1.6 = 600, softcap drops that to 530. You'd move an extra 155 units per second for 3 seconds, 465 in total, assuming no slows). Most dashes cover a greater distance than that in a fraction of the time. Ghost is one of the few non-kit-based increased MS choices that has a long enough duration to actually render it better at covering distance than a typical dash, and even that's not by much (for the 375 MS mentioned above, ghost allows you to cover an additional 890 units over 10 seconds assuming no slows) and it takes a damn long time to do so.
And finally, 'mobile' champs are fully capable of benefiting from those increased MS options as well, as a level of mobility that would be considered 'enough' or 'wastefully excessive' pretty much doesn't exist. On the other hand, the means I mentioned for adding onto the burst of a character lacking burst are not always the best choices for champs that already possess high burst... there is such a thing as having 'enough' or 'wastefully excessive' burst damage. Pretty much this comes down to is the 'haves' of mobility get to benefit just as much as the 'have nots' of mobility from the various increased movespeed options that exist, while the 'haves' of burst generally don't have as much to gain as the 'have nots' of burst when it comes to itemizing burst damage.
Having low mobility is MUCH harder and less effective to buy your way out of than having low burst.